1. The surface region of a fold about a hinge line, which occupies the area of maximum curvature.

2. In Brachiopoda and lamellibranchs, the area of the commissure where the two valves of the shell are permanently in contact. In those articulate brachiopod (Articulata) shells where the whole width of the posterior part of the commissure remains in contact during opening, the shell is said to be ‘strophic’. In those brachiopods where the hinge axis passes through the teeth and sockets, and these represent the fulcra, the shell is said to be ‘non-strophic’.

Hinge