Abstract: An improvement in casement windows provides a mullion portion which is removable to allow for emergency egress through the window, a pivotably disengagable connection being formed between one end of the mullion portion and the head jamb of the window frame, and a tongue and groove connection being formed between downwardly and outwardly inclined complementary surfaces on the other end of the mullion portion and the sill of the window frame that allows the mullion portion to slide outwardly from a position seated in the frame to effect separation of the mullion portion from the frame simply upon opening of a sliding latch.