A coincident constraint forces two points, or a point and line, to coincide.
In the example below, a coincident constraint has been applied to the midpoints of the two lines on the left. The result is the block definition on the right, where the midpoints coincide with each other.
In the example below, the constrained block on the left has been manipulated to produce the block on the right. The two lines can move and change shape, but will always be constrained to coincide at their midpoints.