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26. The Two-piece Elbow.—A model of a two-piece elbow, Fig. 76, can be constructed from a cylindrical piece of wood such as a broom handle. The handle should be cut through at an angle and the two pieces put together so that they will form an angle similar to that shown in the elevation of Fig. 77. It should be noticed that the cut portions are not circles but that the section is longer in one direction than in the other. The two pieces fit together perfectly to form an elbow.

The following facts concerning all elbows are illustrated in Fig. 77. They should be memorized by the student.

The Base Line.—Every elbow is represented as starting from a horizontal line. This line is called the base line of the elbow.

Arcs of the Elbow.—Every elbow is made around the arcs of two circles. These arcs have the same center.

Center of the Elbow.—The center of the arcs around which the elbow is made is also the center of the elbow.

Throat of the Elbow.—That part of the elbow drawn around the smaller arc is the throat of the elbow, and the arc is the arc of the throat.

Back of the Elbow.—That part of the elbow made around the larger arc is the back of the elbow, and the arc is the arc of the back.

Throat Radius.—The throat radius is the distance measured along the base line, from the center of the elbow to the throat.

Center Line Radius.—The center line radius is the distance, measured along the base line, from the center of the elbow to the center line of the big end.

The Backset of an Elbow.—The backset is the amount the back rises (sets) above the throat. This vertical distance is indicated by the dash line drawn horizontally from the highest point of the throat of the big end.

Number of Backsets.—The first piece of an elbow has one back-set, the last piece has one, and every other piece in the elbow has two backsets.

Rule for the Number of Backsets.—The number of backsets is equal to the number of pieces in the elbow less one, multiplied by two. A four-piece elbow would have (4-1)&times;2=6 backsets.