The Meprolight Tru-Dot Green Tritium 4-Dot Fixed Rear Sight is a direct drop-in replacement aperture for AR-15, M16 A1, and M16 A2 rifles, featuring four self-illuminating Swiss tritium dots positioned at the top, bottom, left, and right of the peep aperture so the shooter can instantly center the front sight post within four green reference points for fast, precise target acquisition in any lighting condition. This rear aperture-only sight fits directly into existing A1 and A2 sight housings with no modifications, and pairs seamlessly with a compatible Meprolight Tru-Dot front sight for a fully integrated 5-element tritium sight system used worldwide by military and law enforcement. Drop-proof, shockproof, recoil-resistant, maintenance-free, and backed by Meprolight's 12-year tritium illumination guarantee, these sights are always on and always ready right out of the box.