The marking device shown in the drawing is used to establish cutlines for clapboards where they abut trim pieces. I find it to be especially handy for recladding old houses where the casings and cornerboards aren't plumb.
I make the gauge from a scrap of 3/4-in. thick pine, 1-1/2 in. wide. The slot in the center of the gauge should fit the profile of your clapboards just tightly enough to hold a clapboard in place.
To use the gauge, hold your clapboard with the end to be cut running by the casing, and slip the gauge over the clapboard. Press the gauge tightly against the casing, and scribe along the edge of the gauge. Cut along the line, and the clapboard will fit tightly against the trim board.
Duke York, Willimantic, CT