Use the horizontal/vertical command to make a selected planar face (A) parallel to the most similar base reference plane (B).

You can also apply a horizontal/vertical constraint between two keypoints (A), (B) relative to a reference plane.

During the establishment of a horizontal/vertical relationship, only the seed face(s) or the seed keypoint move (C). A target keypoint (B) does not move during a relate operation.

The horizontal/vertical relationship is persisted by default. You can turn the persist option off while in the command.

You can choose the horizontal/vertical command first. Select an alternate alignment plane or use the default base alignment plane. Then select the keypoints.

You can also create the seed select set first and then choose the horizontal/vertical command.