The ACD will prevent "cant" in your scope that would create accuracy robbing changes in your elevation and windage cross-hair settings.  A must for long range shooters.  It comes with mounting instructions.

Inventor/shooter Dave Beisner has taken a tool common to any homeowner's workbench and incorporated it in an inexpensive scope accessory for match and field shooting. An idea that can be traced back to the late 1800's and the English-made Creedmore match rifle. The patented ACD (Anti-CANT Device) is a simple spirit level mounted in an aircraft-quality, two-piece aluminum scope ring that mounts in minutes without special tools. Properly mounted, the ACD allows the shooter to visualize and make adjustments for CANT, which experienced shooters recognize as rotation of a scope's crosshairs off a true vertical/horizontal plane. According to the Marine Corps Sniper Manual, compensating for CANT is critical to accurate long-distance shooting: At 1000 yards, 6 degrees of CANT (visualized as 1 minute on the face of a wristwatch) moves the bullet strike 55 inches from point of aim. Weekend shooters who find varmints a challenge at several hundred yards are no less immune to CANT: 6 degrees of CANT at 100 yards gives the critters a new lease on life by a full ½ inch. Installed on a properly mounted rifle or pistol scope, the ACD uniquely equips the shooter to visualize and eliminate CANT with 100 percent certainty, 100 percent of the time.

The ACD is a remarkably effective but simple and inexpensive means to improve shooters' accuracy. It consists entirely of a simple carpenter grade bubble level mounted securely in one half of a 2-pc. anodized aluminum scope ring. The ACD is a lightweight ½ oz. And, properly installed, will not shift under recoil. It is extremely durable under regular use in matches and the filed, and has no moving parts that might wear or shift. No special tools, tiny fingers or brute strength required for installation.