In traditional timber framing and joinery, this is the flat face of the mortised member against which the cut shoulders of the tenon rest. It provides the primary resistance against racking and compressive forces in the joint. A tight, gap-free fit between the tenon shoulder and this surface is critical for the structural integrity of the frame.
Heard on the job
Make sure the butment cheek is perfectly flat, or the tenon shoulders will show a gap when we drive the pegs home.