This is a full sized template for tracing out a scarf joint on the end of a timber. There is a template for a 3″ piece and also for a 3-1/8″ piece.
The scarf joint comes in many shapes and varieties, but the main purpose is the same: to join two timbers together end to end to make a longer one. In framing this joint would be placed on top of a post, to prevent any loss of strength in the vertical load direction. This joint is best cut by hand tools. There is a notch in the middle for placing a wedge (or 2) to drive the two halves tighter together.