This old boatbuilder’s trick allows you to draw large arches without using a giant compass or trammel. All you have to know is the arch’s height (also called its rise) and length (its run). You don’t need a center point. I like the long, shallow arches on Mission furniture, so I use this method for making router templates. I prefer 1/4-in. MDF for the jig and template, but any material will do.
Drive nails into the template at the center and the end of the arch. Position the jig with the hypotenuse against the end nail and the flat against the center nail. Hold a pencil in the notch and advance the jig while bearing against both nails. To complete the arch, flip the jig, move the end nail to the opposite end and repeat the process.