A hinge line is 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.[1][2][3]
![](http://upload.wikimedia.org/wikipedia/commons/thumb/f/f5/Mytilidae_hinge.jpg/220px-Mytilidae_hinge.jpg)
The hinge teeth, structures which control the articulation of the valves, are often but not always situated along the hinge line.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/a/a0/Arcidae_hinge.jpg/220px-Arcidae_hinge.jpg)