A T-square is a technical drawing instrument used by draftsmen primarily as a guide for drawing horizontal lines on a drafting table. It may also guide a set square to draw vertical or diagonal lines. Its name comes from its resemblance to the letter T. T-squares come in varying sizes, common lengths being 18 inches (460 mm), 24 inches (610 mm), 30 inches (760 mm), 36 inches (910 mm) and 42 inches (1,100 mm)
Ames Lettering Instrument If you have many lines of lettering to do, you will find a lettering instrument, such as the Ames lettering instrument, shown in figure 3-43, view B, quite useful and timesaving. The top-left section of figure 3-43, view B, shows how to use this instrument in conjunction with a T square to draw properly spaced horizontal guidelines. You insert the point of your pencil through one of the holes, and the instrument slides along the T square as you move the pencil across the page. The enlarged drawing of the instrument in the lower part of the figure shows the details of how the instrument is used. Notice the three rows of holes in the circular disc of the instrument.
An architect's scale is a specialized ruler designed to facilitate the drafting and measuring of architectural drawings, such as floor plans and orthographic projections. Because the scale of such drawings are often smaller than life-size, an architect's scale features multiple units of length and proportional length increments.