Coordinate Systems / Scales > Align Horizontally, Line (Manage Menu)

This command aligns the rotation of the active window's coordinate system to a given line in the drawing so that that edge is being displayed horizontally.

Any line in an existing object can be identified as an edge, including a side of a rectangle or a circle segment leg.

After the line has been identified, the coordinate system's rotation will be aligned to it, using an angle between -90 and 90 degrees.