The thumbhole is the concave area close to the nose comb where the thenar eminence of the 1st finger (i.e., the muscular base of the thumb at the 1st metacarpal) resides with proper gun handling. The thumbhole really dictates hand placement and ensures hand placement is repeatable and consistent. The nose comb to breech face measurement somewhat reflects the thumbhole measurement but I believe the geometry of the actual thumbhole has importance too.

Socialism is almost the worst.