Abstract: A mechanical latch assembly for a retractable door or window system is described. The latch assembly includes a latch subassembly mountable between inner and outer door handles and a catch subassembly mountable on a support surface. When the retractable door or window is adjusted to a fully closed position, the latch and catch subassemblies may be releasably coupled together by engaging a latch and a catch bar. A safety catch may be optionally deployed for maintaining the latch in an engaged position. The latch subassembly may include a pair of rotatable cassettes and a corresponding pair of thumb levers for rotating the cassettes against the bias of a spring to move the latch from the engaged to a disengaged position. The latch may be disposed on one of the rotatable cassettes. The catch assembly may include optional brackets for mounting the catch assembly on a door jamb, pocket door frame, another door handle or other suitable support surface.