The article Key caps contained the following content related to this article topic; no substantive citations were included, which is why I include the content here (see specific version from Nov 2008):
On a keyboard, when one strokes a key, you are pushing on a key cap, that generally pushes down on a rubber membrane this pushes two contacts together and sends a corresponding signal that a key is being pushed. Key caps are usually in two forms. Either pre printed (most common) or have an inset place for a slip of paper to be inserted to mark the function of the key. They are generally held to the keyboard by either with two clips that to be released the keyboard must be opened. The other common type is attachment method of key caps is to be held in place by friction. Usually by cross shaped pegs. This is a less common method.
--User:Ceyockey (talk to me) 02:12, 17 May 2009 (UTC)