Wikipedia
Hinge line
A hinge line an imaginary longitudinal line along the dorsal edge of the shell of a bivalve mollusk where the two valves hinge or articulate. The hinge line can easily be perceived in these images of a mussel shell and an ark shell.
The hinge teeth, structures which control the articulation of the valves, are often but not always situated along the hinge line.
Usage examples of "hinge line".
Instead of striking the rock wall, he hit the door along the hinge line, shearing wood and hinges from shoulder height to floor.