Abstract: The present invention provides an improved interlocking rail and sash slot system that optimizes resistance to sash movement and corresponding air leakage during negative pressure loads, improves energy efficiency and aesthetics of the window design, and provides for wood grain option applications. The improved system may include a window sill and a rail extending from the sill. The rail may include a body portion and a head portion. The head portion may be wider than the body portion in an embodiment. The system may further include a window sash defining a slot. The slot may be configured to receive the head portion. The shape of the head portion of the rail may promote engagement of the head portion with the slot in order to lock the window sill and window sash together when the window is in a closed position.