With an engineers square or try square, one trammel head can be used as a substitute for a marking gauge. Marking gauges are used to draw a line parallel to the edge of a surface.
Step 1 – Install pencil in trammel head
Install a pencil or other scribing device in the body of the trammel head.
Step 2 – Clamp trammel head onto engineer’s square or try square
Clamp the trammel head onto the blade of the square a set distance from the edge of its stock.
Step 3 – Run square along edge
Position the square on the edge of the workpiece.
Run it along the edge of the surface you will be marking, so that the pencil installed in the trammel head marks a line down the length of the part.
The line you have drawn will be parallel to the edge of the workpiece.