which the elbow is to be joined. The patterns shown in Fig. 88 should be separated sufficiently to allow for a lap between each piece and must be so drawn by the student. The laps should be ¾ in. wide. Rivet holes on all sides of each piece should be shown. Figure 90 shows the first and second pieces of an elbow after being "fitted." The throat is "laid off" with the stretching hammer, and the back is "drawn in" with a mallet or raising hammer. By
this method the miter cuts of each piece are "butted" and the true curvature of the elbow preserved.
30. Related Mathematics on Elbows.—Problem 13A.—Backsets of an Elbow.—(See description of Fig. 77 for rule.)
(a) How many backsets has a four-piece, 90° elbow?
(b) How many backsets has a six-piece, 75° elbow?
(c) How many pieces has an elbow having fourteen backsets?
(d) How many miter lines has a six-piece elbow?
Problems on the Rise of the Miter Line.—An elbow is always
