Daotui Sun (倒退榫): Fitting a Door Sill Between Columns That Already Stand

The problem

The sills of a door frame are fitted between columns that are already fixed in place, so the sill cannot simply drop in with a tenon at each end.

One long, one short

The two tenons are made unequal: the long one at least twice as long as the short one. The mortise in each column is cut as deep as its tenon or a little deeper, and the long end’s recess must reach past the clear span between the columns by a little more than the short tenon’s length.

In, then back

The long end goes in first and is pushed home; the sill is then lined up with the other mortise and drawn back the other way, so the short tenon enters. The gap left at the long end is packed tight with a wooden block. Ma Bingjian describes this for the middle and top sills; the bottom sill, which sits over the door-pillow stones, is instead dropped in from above.

Proportions in the model are illustrative.