Keystone (cask)

A keystone is a small wooden or plastic fitting used in ale casks. For some years wooden casks have been effectively obsolete, with the majority now being aluminium or stainless steel, and a few sturdy plastic ones beginning to appear. Nevertheless, the standard tap is still designed to be hammered into yielding material, punching a hole as it goes. All modern casks, even the few wooden ones still in use, use a replaceable keystone to accept the tap.