Many combination squares have a center head. Hold it against any circular or curved surface. The blade’s edge will point directly through the center of a circle, Figure 12-14B. The protractor head adjusts for any angle. You may wish to remove the handle and center head when using the protractor head. See Figure 12-14C. Using an Octagon Scale The octagon scale helps you identify critical measurements for laying out octagons. Suppose you wanted to create an octagon tabletop 28″ across, Figure 12-16. To produce this tabletop, proceed as follows: 1. Cut a piece of material 28″ square. 2. Draw centerlines AB and CD. 3. Set a compass or divider for 28 dots along the octagon scale. 4. Mark the distance on each side of the center- lines along the four edges of the board. 5. Connect the newly marked locations to form the eight sides. Procedure The L.S. Starrett Co. Figure 12-12. Try squares are for both marking and checking squareness. Marking Checking Squareness 90 ° 90 ° 90° The L.S. Starrett Co. Figure 12-13. Parts of a combination square. Level Scriber Square head Protractor head Lock bolt Center head Blade/rule 12.2.4 Tables Tables provide helpful information for com- monly used measurements. Two such tables are the brace measure table and the octagon, or eight-square scale. The brace measure table gives diagonal mea- surements that show the length needed for a diag- onal piece, such as a brace, to support a shelf. The measurements are on the tongue of most framing squares, Figure 12-15. For example, suppose you have a 22″ wide shelf and you wish to brace it at a point 18″ from the wall and 24″ below the shelf. Find the measurement on the table marked 18/24. You will find the number 30 next to it. This is the proper length of the brace. 154 Section 2 Design and Layout Copyright Goodheart-Willcox Co., Inc.