A tee-piece for a square pipe placed diagonally is shown in Fig. 29. The hole on the main pipe is easily marked out when we remember that its length must be equal to the two sides of the pipe and its width equal to the diagonal of the pipe. This is shown projected on the top portion of Fig. 29.

The pattern for the branch pipe is set out by taking the four sides of the pipe for its total girth, and the line A C equal to half the diagonal of the pipe or equal to the length of the line A C on the hole. It will be readily seen that the pattern can be marked out from the hole, or, which is perhaps better, the hole marked from the pattern, if this be made first.