An aircraft seat assembly (14) is convertible between three seats (LA, LM, LW) with a first seat width and two seats (LA, W) with a second wider seat width. The seat assembly includes a tube-in-tube support frame for the seats which allows the conversion to be accomplished by a simple sliding movement of the aisle and middle seat frames towards and away from the window seat frame which is fixed in position. Movement of the aisle and middle seat frames automatically changes the position of the two inside armrests (22, 24). The middle seat back frame (66) is vertically split into parts (66A, 66B). Movement of the seat frames from the three-seat configuration to the two-seat configuration disconnects seat back parts (66A, 66B) from each other and locks one part (66A) to the window seat back frame (64) and the other part (66B) to the aisle seat back frame (68). Movement of the seat frames in the opposite direction disconnects the parts (66A, 66B) from the window and aisle seat back frames ( 64, 68) and connects the two parts (66A, 66B) together to provide a unified seat back frame (66) for the middle seat (LM). A lock mechanism (124, 130) locks the seat frame parts together, both in the three-seat configuration and in the two-seat configuration. The lock mechanism is locked and unlocked by movement of a handle (H) located at the aisle end of the aisle seat (LA). A key lock (160) is provided to keep the lock mechanism (124, 130) locked in between authorized and intended movements of the seat frames to convert the seat assembly (14).