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Drafter's ruler (architect's scale)
How to Use a Three-Sided Drafter's Ruler. The drafter's ruler (also called an architect's scale) has three sides with one or two scales along each of its six edges. One scale is measured in inches and fractions of an inch. The other scales are used to make reduced drawings. Using a three-sided drafter's ruler isn't too difficult if you know what to do with it.
Measuring a Line
Determine the scale of the line you want to measure. In most cases, the line is part of a reduced drawing, and the scale of the drawing is indicated.
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Find the appropriate scale on your drafter's ruler. Available scales on a good quality drafter's ruler are 1/8, 3/32, 3/16, 1/4, 3/8, 1/2, 3/4, 1, 1 1/2 and 3 inches to the foot.
Notice that each scale begins with 0 near the end. To one side is a series of marks (gradations) representing one inch, divided into sixteenths. On the other are gradations representing the number of inches or feet represented by 1 inch on the scale. For instance, for a 1/4 scale, a quarter of an inch represents 1 foot so the number 4 on the scale, which is 1 inch away from 0, represents 4 feet.
Position your ruler so the beginning of the line is even with the gradation indicating 0. The end of the line you're measuring will extend past one of the gradations on the scale. For instance, the line may extend past the gradation for 8 feet without reaching the gradation for 12 feet.
Reposition your ruler so the last gradation the line extends past it at the end of the line. Notice that the 0 is no longer at the beginning of the line. Instead the beginning falls somewhere on the 1 inch scale.
Count the number of sixteenths of an inch to the beginning of the line. For instance, if you are using the 1/4 scale, each sixteenth of an inch represents 3 inches, so if the beginning of the line is 3/16 of an inch beyond 0, the line you're measuring is 9 inches longer than the number of feet represented by the gradation at the end of the line.
Drawing a Line
Decide on the scale you want to use. See Step 2 in the above section for available scales.
Position your ruler so the gradation representing 0 is at the location where you want to begin your line. Use your pencil to make a small mark on the paper at this gradation.
Note the other numbers on the scale and determine where the end of the line, after it is drawn, will fall. For instance, if you are using a 1/4 inch scale and will be drawing a line representing 9 feet, the line will fall between the gradations representing 8 and 12 feet.
Reposition your ruler so the mark you made falls somewhere on the 1 inch scale. For instance, if you are using the 1/4 scale and the line you are drawing is to represent 9 feet, position your ruler so the mark falls at the 1/4 inch gradation.
Find the gradation that, combined with the 1 inch scale, represents the end of your line. Make a second small mark on the paper at this gradation.
Set your drafter's ruler aside and use a straight edge to draw a line between the two marks you made.
Always place your drafter's ruler on the paper so the scale you're using is on the side facing away from you. This will help prevent the shadow of your body or the scale from falling across the scale, which may make accurate measurement more difficult.
Never use a drafter's ruler as a guide for drawing lines. This will cause wear on the edges, leading to inaccuracies when using the ruler for measurements.