HyperCard can be extended significantly through the use of external command (XCMD) and external function (XFCN) modules. These are code libraries packaged in a resource fork that integrate into either the system generally or the HyperTalk language specifically; this is an early example of the plug-in concept. Unlike conventional plug-ins, these do not require separate installation before they are available for use; they can be included in a stack, where they are directly available to scripts in that stack.