Many times, a builder needs to mark a constant gap, or distance, from an irregularly-shaped line. An example would be trimming a wing intersection fairing so that there is a constant gap between the fairing and fuselage for a seal. You can do this by measuring every inch, then connecting the dots—or much more quickly using a scrap of material with a hole drilled at the correct gap distance. Drill the hole the size of the tip of a Sharpie fine-tip marker, and the tool is complete!
A short scrap of aluminum, bent to make a handle, is all you need to make this special tool. Dressing the edges makes it slide easier.
The hole should be sized to perfectly fit the marking utensil to be used—in this case, a fine-tipped Sharpie.