An assembly for a frame, comprising: an end unit (12, 14) comprising an end wall (20, 24), a first side wall (22, 26A) extending perpendicularly from the end wall, and a second side wall (22, 26B) extending perpendicularly from the end wall and forming a corner with the first side wall; an elongated rail, comprising a first flange and a second flange together forming an L-shaped cross section; wherein in a first position one end of the rail is seated in the corner, with an end face of the rail against the end wall (20, 24), the first flange flat against the first side wall, and the second flange flat against the second side wall; characterised in that the first side wall has at least one protrusion abutting an outer edge of the first flange in the first position to laterally restrict movement of the rail away from the second side wall parallel to the end wall and the first side wall; the first side wall has at least one tab extending through a slot in the first flange and engaging the first flange adjacent to the slot on a side of the slot away from the end wall, the tab in the first position preventing movement of the rail away from the first side wall parallel to the end wall and the second side wall, while permitting movement of the rail away from the end wall to a second position where the at least one tab aligns with the at least one slot to permit movement of the rail away from the first side wall; and the second side wall (22, 26B) has a hole aligning with a hole in the second flange in the first position, the aligned holes receiving a pin preventing relative movement of the rail and the end unit from the first position to the second position. an end unit (12, 14) comprising an end wall (20, 24), a first side wall (22, 26A) extending perpendicularly from the end wall, and a second side wall (22, 26B) extending perpendicularly from the end wall and forming a corner with the first side wall; an elongated rail, comprising a first flange and a second flange together forming an L-shaped cross section; wherein in a first position one end of the rail is seated in the corner, with an end face of the rail against the end wall (20, 24), the first flange flat against the first side wall, and the second flange flat against the second side wall; characterised in that the first side wall has at least one protrusion abutting an outer edge of the first flange in the first position to laterally restrict movement of the rail away from the second side wall parallel to the end wall and the first side wall; the first side wall has at least one tab extending through a slot in the first flange and engaging the first flange adjacent to the slot on a side of the slot away from the end wall, the tab in the first position preventing movement of the rail away from the first side wall parallel to the end wall and the second side wall, while permitting movement of the rail away from the end wall to a second position where the at least one tab aligns with the at least one slot to permit movement of the rail away from the first side wall; and the second side wall (22, 26B) has a hole aligning with a hole in the second flange in the first position, the aligned holes receiving a pin preventing relative movement of the rail and the end unit from the first position to the second position.