Allows you to create a keypoint line at an angle to another line or curve.
To draw a line at an angle to a curve
icon, or choose Lines
> Line at Angle from the Keypoint Curve Toolbox.
If you click a curved line, the new line is drawn at an angle to the line that passes through the end point of the curve.

(Windows) or
(Mac). The Line
at Angle tool adds a new guideline at that angle.
As you drag the end point, the Line at Angle tool displays the angle in current angular units (degrees by default).