main tube, the standard mounting method is to use scope rings, which are essentially round metal pipe shoes that clamp firmly onto the telescopic sight body. Most commonly, a pair of scope rings are used, though unusually short telescopic sights occasionally do use only a single scope ring. There are also one-piece mounting accessories with two integral rings, called scope mounts, that can even offer "cantilever" or "offset" mounting (leaning off to one end, away from the center). The scope ring size (inner diameter) must correspond closely to the outer diameter of the telescopic sight main tube, or else the telescopic sight would either be loosely mounted, or sustain compressive fatigue due to being clamped too tightly. The three most common ring sizes are: , offers a lower production cost compared to 30 mm main tubes, but allows for less elevation adjustment than what is possible with a 30 mm tube. 30 mm, the most common main tube standard today, and therefore has the widest range of mounting solutions. 34 mm, which has become a new standard main tube size for tactical rifle telescopic sights where more elevation is needed than with a standard 30 mm tube. Scope bases The scope base is the attachment interface on the rifle's receiver, onto which the scope rings or scope mount are fixed. Early telescopic sights almost all have the rings that are fastened directly into tapped screw holes on the receiver, hence having no additional scope base other than the receiver top itself. While this is simple and cheap, it comes with the problem that any misalignment of the screw holes can cause the scope rings to exert bending stress on the telescopic sight body, and often requires the inner edges of the rings to be lapped before the telescopic sight can be safely mounted. Some scope bases, such as Leupold & Stevens's proprietary STD mounts, use socketed bases screw-fastened to the receiver and a twistlock-like interface to secure the accompanied scope rings. An alternative design that has remained popular since the early 20th century is the dovetail rail, which is a straight metal flange with an inverted trapezoid cross-section (similar to the dovetail joint used in woodworking). When mounting a telescopic sight, dovetail-interfaced scope rings can be slid onto the rail at any desired position, and friction-fastened via set screws, or clamped firm with screw-tightened plates called "grabbers". Due to the