A straight segment between two points, with either end configurable.
The Line draws one straight segment. It is the base of the linear family: an Arrow is a Line that starts with a head on it, and a Polyline is a Line that keeps going.
Draw it: press at one end, drag to the other, release.
Nine, for either end: none, open arrow, closed arrow, reversed open, reversed closed, diamond, circle, square, slash. Head size follows Thickness, so a heavier line gets a proportionally heavier head rather than the same small glyph on a thicker stem.
To measure rather than point, Dimension draws the same two-point segment and labels it with its real length.