The Vortex Bubble Level installs on the tube of your scope and shows you if you are canting to the left or the right, thus allowing you to make the most precise long-range shot by ensuring your riflescope if perfectly vertical. When the perfect shot is required, be sure to get the Vortex Bubble Level. It mounts quickly and easily to the riflescope tube. It is available for 30mm, 34mm, 35mm and 1 inch tubes.