5 inches to about 2 inches in another embodiment this depth and length are about 0 75 2 inches having this depth and length between about 1 inch and 2 inches should suffice for most applications in this preferred embodiment the main frame 12 also has a conventional accessory pocket 30 on the lower front edge thereof for receiving a downwardly depending cowling 13 for covering a forward portion of the extant frame 1 as used herein the phrase u201c transversely directed pocket u201d also includes the accessory pocket 30 in some embodiments the invention provides a method of installing a window in one such embodiment the entire original window need not be removed instead one need remove only the sliding vent and glass for example the remainder of the extant frame 1 may be left intact this can also enable the user to leave the original siding 11 inside wall 12 c and sill 12 b intact the frame 12 of the new window may be inserted in the space defined within the extant frame 1 from the outside window opening the window liner 20 with trim 24 will be inserted from the inside and the projection 22 will be aligned with and inserted into the recess 14 calculations from the opening width will determine the variable transverse dimension of the window frame 12 liner 20 combination joining this frame and liner by means of this unique mortise and tenon concept may significantly simplify retrofit installations making such installations less expensive and more cosmetically attractive as will be appreciated by those of ordinary skill in the art the telescopic adjustability of the liner 20 with respect to the window frame 12 accommodates a wide variety of existing aluminum frame 1 widths such that only semiskilled labor is required.