Finger marking gauge
You won't have to search for a straightedge with this always-at-hand technique.
Here's an easy trick for marking rough layouts or dividing boards into approximate cuts. Cradle a pencil between your thumb and index finger, resting it against the middle finger. By sliding the tip of your middle finger along a board's edge, you'll mark a parallel line. Adjust your middle finger's position on the pencil to control distance from the edge. Watch out for splinters.
—From the WOOD® magazine shop