Constrains cursor movement to the horizontal or vertical direction.
In the illustration, a line is drawn using Ortho mode. Point 1 is the first point specified, and point 2 is the position of the cursor when the second point is specified.
Ortho mode is used when you specify an angle or distance by means of two points using a pointing device. In Ortho mode, cursor movement is constrained to the horizontal or vertical direction relative to the UCS.
Horizontal is defined as being parallel to the X axis of the UCS and vertical as being parallel to the Y axis.
In a 3D view, ORTHO is additionally defined as being parallel to the Z axis of the UCS, and the tooltip displays +Z or -Z for the angle depending on the direction along the Z axis.