The anti-cant device fits onto your scope like a scope ring and tells you whether your rifle is level or not. It is that simple. There has been a lot of research done about the cant of a rifle affecting accuracy (a lot of it was done decades ago). This bubble level affixes to the scope of your rifle where it is easy to see. Perfect for those shooting off a mechanically supported position. Also works well as a front sight level for NRA match and long range rifles.
Big Book Catalog, Issue:70, Page:275
Sinclair, Issue:5B, Page:212