A.T. Robertson Commentary Mark 12:36

"David himself said in the Holy Spirit, The Lord said unto my Lord, Sit thou on my right hand, Till I make thine enemies the footstool of thy feet." — Mark 12:36 (ASV)

The footstool (υποποδιον). Westcott and Hort read υποκατω (under) after Aleph B D L.