Dovetail
An angled, trapezoidal slot machined to hold a sight or mount by friction. An older mounting method that predates standardized rails.
A dovetail is a trapezoidal channel cut across the steel, wider at the bottom than the top, that a matching base slides into and wedges against. It holds iron sights and some optic mounts by friction and the interlock of the angled walls, and it is adjusted for windage by drifting the sight sideways in the slot.
Dovetails predate standardized rails and still appear on rimfire receivers, sight bases, and older actions. For repeatable optic mounting and the canted bases long range needs, the Picatinny rail has largely replaced the dovetail, since a friction slot offers no cross slots to index against for return to zero.