The Flat Lying Trammel Set from M.Power Tools is a simple design that’s quite useful for layouts. It’s comprised of two heads that attach to any standard metal ruler or square. The ruler stays in a horizontal position, making it easier to draw circles and straight lines.
It’s more versatile than a compass for drawing circles because the size of your circle is limited only by the length of your ruler. If you replace the pencil with the included cutting blade, you can cut circles out of card stock or veneer.
Another handy use is drawing parallel lines. Attach one head to your square and you can draw a line parallel to the edge of your board simply by running the square along the board’s edge. Or, you could put pencils in both heads to draw two lines parallel with each other.