To Construct a Segment:

1.  Choose the Segment tool if it’s not already active.

2.  Click to locate the first endpoint of the segment. (You can click in empty space, on an existing point, on a path object such as another segment or a circle, or on an intersection.)

3.  Click again to locate the second endpoint.

If you prefer, you can use any of the Straightedge tools by pressing the mouse button at the location of the first point, dragging the mouse, then releasing the mouse button at the location of the second point.

See also
Constructing Straight Objects at Specific Angles
To Construct a Ray
To Construct a Line

Points
Path Objects
Construct Menu