Horizontal takeoff, horizontal landing (HTHL) — is the mode of operation for the first private commercial spaceplane, the two-stage-to-space Scaled Composites Tier One from the Ansari X-Prize SpaceShipOne/WhiteKnightOne combination. It is also used for the upcoming Tier 1b SpaceShipTwo/WhiteKnightTwo combination. A prominent example of its use was the North American X-15 program.