Abstract: A frame particularly for use in a supplemental window which may be installed in an existing window case comprising sections of a linear extrusion forming a rigid body. The elements including a screw receiving channel, a channel adapted to hold sliding pins, a channel adapted to hold a pane and a channel adapted to fasten a compressible gasket to the element. Four such elements fastened together by means of screws form a rigid body surrounding the pane. The outside dimensions of this rigid body are slightly smaller than the inside dimensions of the window case in which it is adapted to be fitted. The rigid body is surrounded by a compressible gasket which is adapted to be compressed against the inside of the window case to form a seal therewith. The sliding pins are adapted to be slid into holes in the window case and to be locked in that position, thus providing a pivotal mounting for the supplemental window.