Defender Security U 9817 Tamper Resistant Sliding Window & Door Lock, 1/4 Inch Max. Fin, Diecast, White, Hex Screws For use on 1/4 in. max. thick fins manufactured to fit on older style, metal-framed sliding windows and doors  usually of single pane glass These locks will not work with modern vinyl windows, as the lock has only a narrow 1/4 in. opening Hex screws prevent accidental opening by children Heavy duty cast metal with tamperproof double hex security screw and wrench