When a branch pipe which is smaller than, and square to, a main pipe, and is also required to fit flush on the back of the main pipe (say, to lie against a wall), then its pattern will be obtained as shown in Fig. 13.
The necessary lines for the hole and pattern are obtained by marking out an end elevation of the two pipes, as seen on Fig. 13. A line 0 6 to touch the main pipe is now drawn, and upon it a semicircle described, this latter being divided into six equal parts, and perpendiculars run up through the division points to meet the main pipe. The girth line 0 0 of the pattern is made equal in length to the circumference of the branch pipe, divided into twelve equal parts, and lines square to it run up through each division point. These perpendicular lines are now cut off equal to the same numbered line measured from the line 0 6 up to the main pipe circle in the elevation. Thus, for instance, the line 1, 1, on the pattern will be the same length as 1" to 1' in the elevation, and so with all the other heights.
The shape of hole in the main pipe can be marked out by drawing a line, 0' 6', made up of the lengths of the arcs 0' 1', 1' 2', etc., from the main pipe circle, drawing perpendiculars through each point, and cutting these off above and below the line 0' 6', equal to the similarly numbered line on the semicircle in the elevation. Thus, to give an example, the line 1' 1 on the hole will be made the same length as the line 1" 1 on the semicircle in the elevation, and so on for all the other lines.