Dovetail rail

A dovetail rail or dovetail mount can refer to several types of sliding brackets found on firearms primarily for mounting telescopic sights. Colloquially, the term "dovetail rail" usually refer to any straight mounting bracket with an inverted trapezoid (dovetail) cross-section (though the hexagonal-profiled Weaver rail and Picatinny rail are also derivative dovetail designs) running parallel to the bore for mounting a scope or diopter sight to a rifle. These are sometimes also called "tip-off" mounts, and allow the user to easily take on or off the sight. "Dovetail mount" can also refer to a dovetail track running perpendicular to the bore (see "Other uses" below).
Dovetail rails for rifles.
Dovetails come in several different types and sizes depending on manufacturer, but the most common are the 11 mm and inch (9.5 mm). Some other less known, but currently commercially available dovetail mounts, are 12, 13, 13.5, 14, 14.5, 16, 16.5, 17 and 19 mm.
While the well standardized picatinny rail mount (and its less standardized predecessor the Weaver rail) is most known in the U.S., many European gun manufacturers offer proprietary scope base receiver mounting systems for their guns, for example Sako has tapered dovetails, Tikka use a 17 mm dovetail, and there are other solutions such as the Blaser Saddle Mount or Recknagel Swing Mount. Dovetail mounts are today mostly found on light recoiling air guns, but can also be found on some modern rifles for hunting and sport shooting using smokeless powder, although other options such as the picatinny rail are becoming more popular.
Some examples of rifles with different types of rails:
Dovetails are also integrated into some rifles most noticeably the L85A2 and A1 variants of the SA80 bullpup rifle for mounting the iron sights and the SUSAT sighting system. But in recent times it has been changed to Picatinny rail in place of it, as Dovetails where not fit for the purpose of making space for ACOG sighting system, according to the MOD 
Side mounted scope rail.
While most dovetail rails are placed on top of the receiver, there are also examples of side-mounted rails. Some service rifles used by Soviet and Warsaw Pact nations armed forces have a distinct type of side-mounted scope, informally known as a . The mount is found on the left side of the rifle receiver, with machined cutouts for reduced weight and ease of installation; an example is the PSO-1 optical sight. Similar rails can also be found on rifles such as the Dragunov sniper rifle (SVD), the PSL rifle, the PKM as well as some AK series assault rifles from 1954 onwards. Since 1992 the side rail mount has become standard on all AK rifles.
The SVD and the AK rifles use slightly different dovetail mounts. The Warsaw rails are roughly 14 mm wide.
Other uses.
"Dovetail mount" can also refer to a dovetail track running perpendicular to the bore, often used for smaller front sights posts and rear sights blades found on handguns and some rifles. This mounting method is meant as a semi-permanent friction fit mounting solution where a slot is milled, for instance in the slide of a pistol, and a sight with a corresponding dovetail is punched or drifted into that slot.
Rear sights are offered in many dovetail cut profiles which are non-compatible, and some well known (and incompatible) cut profiles are those from sight manufacturers such as Novak, BoMar, LPA/TRT, Kimber or the 1911 mil standard ("GI"). Additionally many pistol manufacturers have their own proprietary dovetail cut profiles.