is the smaller of the two disposable, circular bungs on a traditional cask, located centrally at the bottom of the face of the vessel. The standard “No. 2” keystone size measures 3.75 cm (1.5 in) in diameter at the front, tapering down to 3.5 cm (1.4 in) at the rear. A broader size, “No. 1,” has largely been phased out but remains in limited use by a few British regional breweries.

The outer rim, 0.75 cm (0.3 in) wide on the standard fitting, is deeper than the central plug, with the latter part being shallower to facilitate the tap being manually driven through and being held firm by the wider rim. Traditionally, this fitting would be made of limewood and drilled through in slightly different places at the front and back to ensure a good seal prior to being broached, at the same time allowing for the tapping process to be undertaken without excessive effort. Today plastic keystones are becoming increasingly popular because the material is cheap to manufacture; they also have such sanitary benefits as easier cleaning and the inability to attract fungal growth, which can be a problem with wooden fittings.

Limewood keystones can occasionally split their rims on tapping if not inserted completely flush in the cask’s bush or if not pressed in far enough, causing beer to leak out and necessitating a fresh keystone to be inserted promptly. A good cask cellarperson should possess the skill to swiftly replace a split keystone without upending the cask and with minimal beer loss.

For hygiene purposes, it is normal—and courteous—for a sealing bung known as a clip cork to be inserted into the open keystone of a freshly emptied cask as soon as the tap is removed. This prevents insects from entering and breeding and stops potential spillage of yeasty beer dregs on the journey back to the brewery.