This pair of 21 by 36 inch extension springs is made for 6-foot garage doors with closed loop ends. Choose the door weight option that matches the actual weight of your door so the springs can balance the door correctly. The closed loop end style is made for standard extension spring hardware and helps keep the connection stable during normal door travel. These springs are intended for doors that already use an extension spring system, and replacing both sides as a pair helps the door move more evenly.