To draw an octagon of 8 inches square, draw an 8 inch square and then draw a perpendicular and a horizontal line through its center. To find the length of the octagon side, place one point of a compass on any of the main divisions of the scale, and the other point of the compass on the eighth subdivision; then step this length off on each side of the center lines on the side of the square, which will give the points from which to draw the octagon lines.
The diameter of the octagon must equal in inches the number of spaces taken from the square.
On the opposite side of the tongue, in the center, will be found the brace rule ([see Fig. 3]). The fractions denote the rise and run of the brace, and the decimals the length. For example, a brace of 36 inches run and 36 inches rise, has a length of 50.91 inches; a brace of 42 inches run and 42 inches rise, has a length of 59.40 inches; etc.
Fig. 1. Face Side of Tongue of Steel Square, Showing Octagon Scale.
Fig. 2. Back of Blade of Steel Square, Showing Rafter Table.