This spring-loaded MBUS (Magpul back-up sight) rear sight can be easily activated from either side or by pressing on the top. The flip up sight clamps to any Mil-Std 1913 Picatinny rail. Protective wings shield the dual, same plane flip apertures. Detent knob easily and positively adjusts windage. Impact resistant polymer construction resists operational abuse. Weighs 1.3 ounces. MADE IN THE U.S.A.