Home Workshop Techniques: Measuring and Marking

A vital part of many home workshop jobs is the preliminary reading and transferring of measurements. Workpieces must be measured and marked before they can be cut or trimmed, and surfaces need to be measured and marked before fasteners are installed. The measuring and marking tools presented in this section are used to perform all the basic functions typically required for home repairs; refer to the inventory in this section. Included are techniques for measuring and marking distances using a tape measure (ref. 32); measuring and marking angles using a try square (ref. 35), a carpenter’s square, a combination square (ref. 36) or a sliding bevel (ref. 37); and checking horizontal orientations using a carpenter’s level (ref. 39), a line level or a water level (ref. 40), and checking vertical orientations using a carpenter’s level (ref. 39) or a plumb bob (ref. 41). To choose the correct tool for the problem or task at hand, consult the Troubleshooting Guide below.

Measuring and marking can be a precise science, often resembling a geometry exercise. Home repair projects, however, rarely require complex measuring or marking; deviations of 1/16” are usually acceptable. Avoid errors by using the same tool for all measuring and marking done on a job. When possible, avoid measuring; for instance, if marking a piece to be cut to the shape of another, use the original as a template instead of measuring and transferring its dimensions. Care and accuracy pays off when a piece fits as planned—with no waste. Check that corners are square using a try square (ref. 35) or a combination square (ref. 36); check that surfaces are flat using a straightedge (ref. 33). To correct irregularities, see Trimming and Smoothing (ref. 78). Measuring tools are precision instruments. Inspect a measuring tool closely before using it and handle it carefully while using it; after using it, wipe off dirt and moisture with a soft cloth, then store it safely away.

TROUBLESHOOTING GUIDE:

MEASURING and MARKING TOOLS

• Straightedge: For precision marking of straight lines and for checking flat surfaces. Thick, metal edges machined straight; one edge usually beveled.
• Carpenter’s square: For checking or measuring angles on a flat surface; can also be used as a straightedge. Most useful size has 24-inch long arm and 16-inch short arm.
• Carpenter’s level: To check orientation of lines and surfaces. Position of bubble in vial indicates perfect horizontal (level) or perfect vertical (plumb). For general use, a 24-inch long model is recommended.
• Carpenter’s square gauges: Clamp to carpenter’s square to set a specific angle for repeated marking; ideal for marking stair stringers—originally called stair gauges.
• Water level: Determines if points separated by obstacle or at great distance apart are perfectly horizontal (level); operates on principle that water seeks own level.
• Chalk line: Reel dispenses chalk-covered string to mark long straight lines.
• Try square: For checking or marking 90-degree angles. Blade available in 6- to 12-inch lengths; 8-inch model most practical.
• Carpenter’s pencil: Marks straight lines; less practical for curves. Will not roll when set down. Sharpen with a knife.
• Plumb bob: Pointed weight suspended from a string becomes a perfectly vertical (plumb) line; used to indicate or mark plumb lines.
• Tape measure: Measures distance between points along straight lines and across diameter or around circumference of cylindrical objects. A 16 to 25-foot long blade 3/4- or 1-inch wide is recommended for general use.
• Protractor: For checking or measuring angles on a flat surface; can also be used to measure or set specific angles on sliding bevel. For general use, a 6-inch model is recommended.
• Awl: Scribes marking lines on wood with more precision than pencil; use also on materials that don't show pencil mark.
• Utility knife: Tip scores marking lines on wood with more precision than pencil; use also on materials that don't show pencil mark.
• Ruler: For precision measuring or marking of straight lines and points on straight lines; can also be used as straightedge. Most useful size is 24 inches.
• Sliding bevel: Adjusts to copy any angle, then to transfer or measure it. Blade stores in handle.
• Combination square: For checking or marking angles. Available in blade lengths from 6 to 16 inches; 12-inch model most useful. Detachable blade doubles as ruler or straightedge.
• Compass: Pencil clamps to one leg to mark circles or arcs and copy irregular lines; available in different sizes.
• Line level: Determines if long lines or distant points are perfectly horizontal (level); clips onto string stretched taut between end points.
• Trammel heads: Attach to straight stock to draw large circles or arcs. Scribe with tip or mark with pencil clamped to one head.

USING A STRAIGHTEDGE

Measuring a circumference. Prepare to use your tape measure (ref. 32). To measure a circumference, use a blade marking rather than the blade hook as a starting reference point. Holding the blade hook, pull the case to extend the blade, then depress the blade lock. Wrap the blade around the circumference, holding the blade hook away from the surface to prevent interference; then, pull gently until the blade lies flat against the surface, with two edges touching. To calculate the distance in inches, take any two perfectly aligned blade markings and subtract the lesser number from the greater number; in the instance shown, subtract 4 from 15 to find a circumference of 11 inches. Release the blade lock to rewind the blade. An alternate method for finding a circumference is to measure the diameter, then multiply it by 3.141 6.

Inspecting a straightedge. Before using a straightedge, check it for warpage. Lay the straightedge flat on a piece of paper and mark a line along one edge, then turn the straight edge around and align the same edge on the opposite side of the marked line. If the edge deviates from the line, the straight edge is warped and should not be used. Check the other edge the same way.

Checking a surface for flatness. Inspect your straightedge, then place it flat- or beveled-edge down across the surface; repeat the test at several places along the surface at different angles across it. If there are gaps or light is visible between the surface and the straightedge, the surface is not perfectly flat. If necessary, trim or smooth the surface (ref. 78).

Marking a straight line. If necessary, use a tape measure to mark any end point for the line (ref. 32). Inspect your straightedge (step far left), then lay it flat on the surface, aligning its beveled edge with any marked end point. Holding a sharp pencil firmly against the beveled edge, draw lightly along it; for greatest precision and neatness, use a utility knife or a scratch awl the same way.

PREPARING TO USE A TAPE MEASURE

Inspecting and servicing a tape measure. To inspect a tape measure, pull out the blade and depress the blade lock; then, release the blade lock. If the blade does not lock or rewind, replace the tape measure. If the blade is worn or damaged, replace it with an exact duplicate. To remove the old blade, pull it out all the way, ex posing the spring at the end. Depress the blade lock and insert a nail in the spring hole to keep the spring from rewinding. Twisting the end of the blade slightly, detach it from the hooked end of the spring. To install the new blade, hold it firmly, still coiled and with its markings facing uip the slotted end of the blade over the hooked end of the spring, then pull straight back to lock it to the spring. Holding the blade steady with one hand, remove the nail from the spring hole and release the blade lock. Ease the end of the blade into the case, then release it; as the blade rewinds, slow it to keep its hook from hitting the case.

USING A TAPE MEASURE

Measuring and marking an outside distance. Prepare to use your tape measure. Place the blade hook at the starting point for the measurement, butting it against an inside edge or hanging it over an outside edge; if the starting point is not at an edge, measure as you would a diameter (ref. 33). Pull the case to extend the blade, stopping just past the point to which you are measuring, then depress the blade lock; if you are measuring from an inside edge, hold the blade with your finger as you pull the case. Tilt the case so the blade edge is flush against the surface. To take a measurement, read the appropriate blade marking; to mark a measurement, use a sharp pencil to mark a crow’s foot at the appropriate blade marking. Release the blade lock to rewind the blade.

Measuring an inside distance. Prepare to use your tape measure. Place the blade hook at the starting point for the measurement, butting it against an inside edge. Holding the blade steady with your finger, pull the case to extend the blade, stopping when the case is flush against the opposite inside edge; depress the blade lock. To calculate the distance of the enclosed space, read the blade marking just where the blade exits the case, then add to it the case length measurement—usually marked on the side or bottom of the case. On some tape measures, the back of the blade is specially marked for in side measurements; read the marking on the back of the blade just where it exits the case. Release the blade lock to rewind the blade.

Measuring a diameter. Prepare to use your tape measure (ref. 32). a measure a diameter or from a starting point with no well-defined Edge, use a blade marking rather than the blade hook as a starting reference point. Holding the blade hook, place the 1-inch blade mark ‘g. for example, at the starting point for the measurement. Pull the case to extend the blade just past the point to which you are measuring, then depress the blade lock. To measure a diameter, move the blade along the edge of the curve opposite the starting point to find the point at which the distance is greatest. To calculate the stance in inches, subtract the number at the starting point from the number of inches at the end point; in the instance shown, subtract 1 from 4 1/2 to find a diameter of 3 1/2 inches. Release the blade back to rewind the blade.

USING A CHALK LINE

1. Inspecting and servicing a chalk line. Inspect a chalk line before using it. If the string is frayed or broken, replace it (step 2). Check the chalk level in the case; on most models, slide open the latch or unscrew the ca the chalk does not cover the coiled string, buy powdered chalk at a hardware store and refill the case, gently squeezing in enough to just cover the coiled string. When the chalk just covers the coiled string, close the case and use the chalk line to mark a straight line (step 3).

2. Replacing the string. If the string is frayed or broken near the hook, cut off the damaged section and retie the hook. If a large section of the string is frayed or broken, buy an identical string at a hardware store. Open the case latch or cap and empty out the chalk. Unlock the crank and pull out the old string, then cut off the hook and set it aside. Unscrew and separate the sections of the case; note the arrangement of internal components for reassembly. Cut the string off the spool. To fasten the new string on the model shown, thread one end through the hole in the spool and knot it. Pulling the string taut, wind it onto the spool several turns. Position the string at the mouth of the case and reassemble it. Turn the crank to reel in the string, stopping near the end to tie on the hook. Reel in the rest of the string and fill the case with chalk (step 1).

3. Marking a straight line. Use a chalk line to mark a straight line longer than a few feet. If necessary, use a tape measure to mark any end point for the line (ref. 32). To check that marked end points on a vertical surface are perfectly horizontal, use a line level or water level (ref. 40); to check that marked end points on a vertical surface are perfectly vertical, hang the chalk line as you would a plumb bob (ref. 41). Drive a nail partway into the surface at one end point; on a vertical surface, at the took the chalk line over the nail and unreel it to the other end point, keeping it off the surface. Pull the string taut and press it against the end point with the thumb of one hand. If the line is less than 12 feet in length, use the other hand to lift the string near the center and snap it once only against the horizontal or vertical surface. If the line is 12 feet or more in length, have a helper press the string at the center, then snap it twice the same way, once only on each side of the center. Unhook the chalk line and reel it in.

PREPARING TO USE A SQUARE

Choosing and inspecting a square. Choose the square best suited to the measuring or marking job. Any square can be used along an edge to check angles or mark lines at angles; as a rule, a try square is the best size for short lines, a combination square for medium-length lines and a carpenter’s square for long lines. To check angles or mark lines at angles when there is no edge for a reference, use a carpenter’s square—the only square that can be used away from an edge. If you must measure lines you are marking with a square, use a combination square or a carpenter’s square; both have numbered rules. If the blade or arm of a square is not long enough for a job, extend its reach using a straightedge, placing the beveled edge along it.

Before using a square, check that its angle is perfectly square. To check a try square, press the handle flush against a straight edge so the blade rests flat on the surface. Use a sharp pencil to mark a line along the outer edge of the blade, then flip the square over and align the same edge on the opposite side of the line; if it deviates from the line, the square should not be used. To check a combination square or a carpenter’s square, use the same procedure; with the carpenter’s square, press the long arm flush against a straight edge and mark a line along the short arm.

USING A TRY SQUARE

Checking for a 90-degree angle. Prepare to use your try square. To check that an angle formed by two surfaces is 90 degrees, choose one surface as a reference— generally, the surface to which the other surface will be adjusted. Holding the handle of the square flush against the reference surface, butt the blade against the other surface; use the outer edges of the square to check an inside angle and the inner edges of the square to check an outside angle. If the blade and the handle fit flush against the surfaces, the angle is 90 degrees. If there is a gap or light is visible between the blade and the surface, as shown, the angle is not 90 degrees.

Marking a line at 90 degrees to an edge. If necessary, use a tape measure to mark any end point for the line (ref. 32). Prepare to use your try square, then press the handle flush against the edge with the blade flat on the surface, its outer edge aligned with any marked end point. Holding a sharp pencil firmly against the outer edge, draw lightly along it; for greatest precision and neatness, use a utility knife or a scratch awl the same way.

USING A CARPENTER’S SQUARE

Marking a line at a 90-degree angle. If necessary, use a tape measure to mark any end point for the line. Prepare to use your carpenter’s square (ref. 35). To mark a line at 90 degrees to an edge, press the inner edge of the long arm flush against the edge with the short arm flat on the surface, its outer edge aligned with any marked end point. Holding a sharp pencil firmly against the outer edge, draw lightly along it; for greatest precision and neatness, use a utility knife or a scratch awl the same way. To mark a line at 90 degrees to another line, follow the same procedure, laying the square flat and aligning the inner edge of the long arm with the line.

Marking a line at a 45-degree angle. If necessary, use a tape mea sure to mark any end point for the line. Prepare to use your carpenter’s square (ref. 35). To mark a line or repeated lines at 45 degrees to an edge, attach gauges to the carpenter’s square, screwing one to the outer edge of each arm at the same distance from the corner. Hold the carpenter’s square so the gauges are flush against the edge with the arms flat on the surface, the outer edge of the appropriate arm aligned with any marked end point. Holding a sharp pencil firmly against the outer edge, draw lightly along it; for greatest precision and neatness, use a utility knife or a scratch awl the same way.

USING A COMBINATION SQUARE

Checking for a 45- or 90-degree angle. Prepare to use your combination square (ref. 35). To check that an outside angle formed by two surfaces is 45 degrees, use the 45-degree face of the handle and the inner blade edge; if necessary, loosen the lock nut and adjust the blade position, sliding it in either direction, then tighten the lock nut. Choose one surface as a reference—generally, the surface to which the other surface will be adjusted. Holding the 45-degree face of the handle flush against the reference surface, butt the blade against the other surface. If the blade and the handle fit flush against the surfaces, the angle is 45 degrees. If there is a gap or light is visible between the blade and the surface, as in the instance shown, the angle is not 45 degrees.

To check for other angles with the combination square, deter mine the face of the handle and any blade edge to be used, then adjust and use the square following the same procedure. For an outside 90-degree angle, use the 90-degree face of the handle and the inner blade edge. For an inside 90-degree angle, use the 90-degree face of the handle and the outer blade edge. For an inside 45-degree angle, remove the blade, then use the 45-degree face and the 90-degree face of the handle.

Marking a line at 45 or 90 degrees to an edge. If necessary, use a tape measure to mark any end point for the line (ref. 32). Prepare to use your combination square (ref. 35). To mark a line at 45 degrees to an edge, use the 45-degree face of the handle and the outer blade edge; if necessary, loosen the lock nut and adjust the blade position, then tighten the lock nut. Press the handle flush against the edge with the blade flat on the surface, its outer edge aligned with any marked end point. Holding a sharp pencil firmly against the outer edge, draw lightly along it; for greatest precision and neatness, use a utility knife or a scratch awl the same way. To mark a line at 90 degrees to an edge, follow the same procedure, using the 90-degree face of the handle and the outer blade edge.

Marking a line parallel to an edge. If necessary, use a tape measure to mark any end point for the line (ref. 32). Prepare to use your combination square (ref. 35), then press the 90-degree face of the handle flush against the edge and loosen the lock nut; ad just the blade position, aligning its end with any marked end point or to the set distance from the edge, then tighten the lock nut. Holding a sharp pencil firmly against or in the notch at the end of the blade, slide the handle along the edge; for greatest precision and neatness, use a utility knife or a scratch awl the same way.

USING A SLIDING BEVEL

1. Copying an angle. Use a sliding bevel to set (step 2) or copy an angle. To copy an angle, loosen the blade — usually by unscrewing a wing nut. For an in side angle, hold the outer edge of the handle against one surface, then swing the blade so its outer edge lies flush against the other surface. Set the bevel by tightening the wing nut. For an outside angle, follow the same procedure, using the inner edges of the handle and the blade. Measure the angle, if necessary (step 2), then mark it (step 3).

2. Setting or measuring an angle. To set or measure an angle, hold the inner edge of the handle against the base of a protractor, aligning the inner edge of the blade with its center point. To set an angle, loosen the blade—usually by unscrewing a wing nut; then, swing it until its inner edge intersects the desired numerical point on the protractor scale and tighten the wing nut. To measure an angle, read the numerical point on the protractor scale intersected by the inner edge of the blade.

3. Marking a line at an angle to an edge. If necessary, use a tape measure to mark any end point for the line (ref. 32). To mark the line at the angle of the sliding bevel, press the inner edge of the handle flush against the edge with the blade flat on the surface, its outer edge aligned with any marked end point. Holding a sharp pencil firmly against the outer edge, draw lightly along it; for greatest precision and neatness, use a utility knife or a scratch awl the same way.

USING A COMPASS OR DIVIDERS

Marking a circle. If necessary, use a tape measure to mark a center point for the circle (ref. 32). To mark a large circle, set up trammel heads (step 1). Otherwise, use a compass or dividers. To set up a compass, close its legs, then loosen the thumbscrew and insert a sharp pencil into the holder. Align the end of the pencil with the tip of the other leg, then tighten the thumbscrew. Using a ruler as a guide, turn the lock nut to open the legs to the radius of the circle. Holding the compass by its handle, position the tip on the marked center point. Turn the handle to rotate the compass on the tip, drawing the pencil along the surface in a smooth curve around it. Set up and use dividers the same way.

Copying a line from a curved or irregular edge. To copy a curved or irregular edge, use a compass or dividers. To set up a compass, close its legs, then loosen the thumbscrew and insert a sharp pencil into the holder. Align the end of the pencil with the tip of the other leg, then tighten the thumbscrew. Butt the surface to be marked against the edge to be copied, as shown, and find the point where the gap is largest: turn the lock nut to open the legs to the distance. Holding the compass with the tip on the edge to be copied and the pencil on the surface to be marked, draw the tip along the edge, marking an identical line on the surface with the pencil. Set up and use dividers the same way.

USING TRAMMEL HEADS

1. Setting up trammel heads. To mark a small circle, set up and use a compass or dividers (step above, left). Otherwise, set up a beam-type compass using trammel heads and a straight, rigid piece of wood or metal slightly longer than the circle radius and no wider than the trammel head clamps. If one trammel head holds a pencil, loosen its thumbscrew and align the end of a sharp pencil with its tip, then tighten the thumbscrew. Attach a trammel head near one end of the beam, turning its thumbscrew to clamp it. Using a ruler as a guide, position the other trammel head on the beam so the distance between the trammel head tips is equal to the radius of the circle: then, tighten its thumbscrew to clamp it.

2. Marking the circle. If necessary, use a tape measure to mark a center point for the circle (ref. 32). Holding the beam steady by its ends, position the trammel head without a pencil on the tip of the marked center point for the circle. Support the beam firmly near the trammel head at the center point for the circle and slowly move the end of the beam at the other trammel head, rotating it on the tip of the trammel head and drawing the pencil along the surface in a smooth curve around it.

USING A CARPENTER’S LEVEL

Inspecting and servicing a carpenter’s level. Inspect a carpenter’s level before using it. If a vial is broken, try to replace it. If the vial is molded to a casing, pry it out and snap in a replacement. On other models, unscrew the vial cover, then replace the vial and put back the cover. To check the level, stand it level on one of its long edges; note the bubble position in the horizontal vial. Turn the level 180 degrees and stand it on the same edge at the same spot; the bubble position should be the same. Check the other long edge the same way. If the bubble position is not the same for each reading, the level should not be used.

Marking and checking for level or plumb lines and points. If necessary, use a tape measure to mark an end point for any line (ref. 32). Prepare to use your carpenter’s level (step left). To mark a level (perfectly horizontal) line on a vertical surface, hold the level flat against it and align one end of a long edge with any marked end point or reference point; then, move the other end slightly up or down, examining the bubble in the horizontally-oriented vial. When the bubble is exactly centered in the vial, use a sharp pencil to mark points along the long edge, then use a straightedge to mark a line between the points (ref. 33).

To mark a plumb (perfectly vertical) line on a vertical surface, use the same procedure, holding the level vertical and examining the bubble in the horizontally-oriented vial. Mark a point that is level or plumb with another point on the same surface the same way. To check that a marked line is level, hold the level flat, aligning one long edge with the line. Examine the bubble in the horizontally- oriented vial; if it's exactly centered, the line is level. To check that a line is plumb, use the same procedure, holding the level vertical and examining the bubble in the horizontally-oriented vial.

Checking for a level surface. Before checking that a horizontal surface is level (perfectly horizontal), check that it's flat (ref. 33); if necessary, trim or smooth it (ref. 78). To check that a flat surface is level, prepare to use your carpenter’s level (step above, left), then stand one long edge across the center of the surface and examine the bubble in the horizontally-oriented vial. Turn the level 90 degrees and stand it on the same edge across the same spot; examine the bubble in the horizontally-oriented vial again. If the bubble is exactly centered for both readings, the surface is level.

Checking for a plumb surface. Before checking that a vertical surface is plumb (perfectly vertical), check that it's flat (ref. 33); if necessary, trim or smooth it (ref. 78). To check that a flat surface is plumb, pre pare to use your carpenter’s level (step above, left), then hold one long edge against the surface and examine the bubble in the horizontally- oriented vial. If the bubble is exactly centered, the surface is plumb. To check that a vertical object is plumb, check two adjacent surfaces the same way; if each surface is plumb, the object is plumb.

USING A LINE LEVEL

Marking and checking for level. If necessary, use a tape measure to mark an end point for any line (ref. 32). To mark a point level (perfectly horizontal) with another point, use a line level and a strong string long enough to reach from the marked end point or reference point. Attach the string at the marked end point or reference point, then pull it taut to the surface to be marked and have a helper hang a line level on it at the midpoint. With your helper examining the bubble in the vial, hold the string against the surface and move it slightly up or down. When the bubble is exactly centered, steady the string and use a sharp pencil to mark a level point along it.

To check that two points are level, use the line level and the string the same way, attaching the string at one point and pulling it taut to the other point; if the bubble is exactly centered, the points are level. To mark a level line between level points on the same surface, use a chalk line (ref. 34). To check that a line is level, follow the same procedure with the line level and the string, attaching the string at one end point and pulling it taut to the other end point; if the bubble is exactly centered, the line is level.

USING A WATER LEVEL

1. Setting up a water level. Set up a water level with a hose long enough to reach between points being marked or checked. Attach the hose to each cylinder base; on the model shown, fit it onto the spigot. Stand the cylinders on a level surface and uncap one. Mix a little food coloring with water, then pour it into the uncapped cylinder until it reaches the zero mark on each cylinder scale; tap the hose and the cylinders to release trapped air. Then, recap the cylinder. Hold a Thumb over each cap hole to carry the cylinders.

2. Marking and checking for level. If necessary, use a tape measure to mark an end point for any line (ref. 32). To mark a point level (perfectly horizontal) with another point, have a helper align an edge of one cylinder base with the marked end point or reference point. Hold the other cylinder against the surface and move it slightly up or down. When the water level is at the zero mark on each cylinder scale, the cylinder bases are level; use a sharp pencil to mark a level point along the edge of the cylinder base corresponding to the one aligned by your helper.

To check that two points are level, use the water level the same way, aligning a cylinder base at each point; if the water level is at the zero mark on each cylinder scale, the points are level. To mark a level line between level points on the same surface, use a chalk line (ref. 34). To check that a line is level, follow the same procedure with the water level, aligning a cylinder base at each end point; if the water level is at the zero mark on each cylinder scale, the line is level. To check that a surface is level, stand each cylinder at opposite ends, then at opposite sides; if the water level is at the zero mark on each cylinder scale for both readings, the surface is level.

USING A PLUMB BOB

Marking and checking for plumb. Before using a plumb bob, check the string; on the model shown, it should pass through the top and out the side of the bob, then be knotted. If necessary, use a tape measure to mark an end point for any line (ref. 32). To hang a plumb bob, work with a helper and use a ladder, if necessary. To mark a point plumb (perfectly vertical) with another point on another surface, hold the string near the base of the bob at the marked end point or reference point. Slowly release the string, letting it slide through your fingers: stop when the bob tip is about 1/8” from the surface to be marked. If the bob swings, slow it. When the bob is still, have a helper use a sharp pencil to mark the point directly below its tip, making an X (inset, left). To check the mark, lower the bob so its tip just touches the surface: if it rests at the X center, the mark is plumb.

To mark a point plumb with another point on the same surface, drive a nail partway into the surface at the marked end point or reference point, then loop the string near the base of the bob over it and slowly release it until the bob tip is low enough for the surface to be marked. Tie the string to the nail; to keep it off the surface, wedge a 1-inch wood block behind it just below the nail. Prepare to use your combination square (ref. 35), then hold the 90-degree face of the handle against the surface with the blade just touching the string—but not deflecting it. Using a sharp pencil, mark a point along the edge of the handle directly behind the string.

To check that two points are plumb, use the plumb bob the same way. To mark a plumb line between plumb points on the same surface, use a chalk line (ref. 34). To check that a line is plumb, follow the same procedure with the plumb bob and the combination square. To check that a surface is plumb, use the plumb bob and the combination square, measuring the distance between the surface and the string at a number of points: if each measurement is the same, the surface is plumb.

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