Default UI Menu: Constraints/Coincident

Ribbon UI Menu:

Makes a vertex lie on another vertex, or on another object.

1. Start with two lines. Activate the Coincident constraint and select the vertex you want to constrain.

1. Select the line which you want the vertex to lie on.

The vertex moves to the line, or to a point on the line's theoretical extension.

If you constrain the vertex to another vertex . . .

. . . the two vertices will meet.

1. This constraint is similar when using arcs and circles. Start with a line and a circle, and constrain the line endpoint to the arc endpoint.

The endpoints now meet.

1. If you constrain the vertex to the arc itself . . .

. . . the line joins the arc at the tangent point.