Compact bubble level instantly displays the amount of tilt or "cant" as you hold your rifle so you can make fast, easy corrections to improve accuracy. Installs on either side of rail/optics to accommodate both left- and right-hand shooters. Folds out-of-the-way when not in use, and a knurled aluminum cover threads over sighting bubble to protect it. Rail Mount clamps to a MIL-STD 1913 Picatinny rail. Protective cover threads onto opposite side for storage when not in use. Mfg: Us Optics SPECS: 6061 T6 aluminum, Type III hardcoat anodized, matte black. Green liquid. Rail Mount - 5/8" (1.6cm) long, 3A 1/4" (8.3cm) wide open, 17/8" (4.8cm) wide folded, A 1/2" (1.3cm) high.