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Oops, problem. Text annotations are typically about 3/32″ or 1/8″ high. Now, if you draw your 6-inch–wide wall full size, put a 1/8″-high title beside it, and then print the drawing at a scale of 1:24, the wall itself will measure 1/4″ on the sheet, and the note will be an illegible little speck beside it. You fix it by making the text 24 times larger, or 3 inches tall, so that it scales down to the correct size when plotted. See, sometimes two wrongs do make a right. Unusually, it takes three, although three rights make a left and two Wrights made an airplane.

CAD users employ two different ways of talking about a drawing’s intended plot scale: drawing scale and drawing scale factor.

Drawing scale is the traditional way of describing a scale: “traditional” because it existed long before CAD came to be. Drawing scales are expressed with an equal sign or colon: for example, 1/8″ = 1′0″, 1:20, or 2:1. You can translate the equal sign or colon as “corresponds to.” In all cases, the measurement to the left of the equal sign or colon indicates a paper measurement, and the number to the right indicates a real-world measurement. A metric drawing scale is usually expressed without units, as a simple ratio. Thus, a scale of 1:20 means 1 unit on the plotted drawing corresponds to 20 units in the real world.