Wooden Keystone

A Keystone is pounded into the cask (firkin) top (outlet) before being tapped.  The keystone is pre-scored to allow the tap to drive into the cask.  A benefit of a wooden keystone over plastic is its moldability to the firkin opening.  Quite often through use the opening in the firkin is damaged and mis-shapen and the wooden keystone is able to mold itself to the opening.