Measuring and Fitting Tenons

The mortise and tenon is one of the oldest and most common methods of joining two pieces of wood at a perpendicular (or near perpendicular) angle.  A mortise is the hole in a piece of wood (think mortar), and a tenon is the piece of wood that lives in the mortise (think tenant).

A tenon must be smaller than the mortise it is entering, but the tenon must also fit snugly enough inside its mortise that the tenon will not pull back out after the glue dries.  I mill my tenons to be .003" smaller than their mortises because I have found this to be an ideal compromise between easy entrance and a snug fit.  Typically, I saw my tenons to be slightly larger than my mortises, and then I plane down the tenons removing .001" of wood per pass until the tenon is .003" smaller than the mortise and fits perfectly.

(NCTM Standard: Number and Operations)