XML in a Nutshell, 3rd Edition

Name

key( )

Synopsis

                     node-set key(string keyName, string value)
node-set key(string keyName, node-set values)

The key( ) function returns a node-set containing all nodes in the source document that have a key with the name given by the first argument and the value given by the second argument. If the second argument is a node-set, then the node-set returned contains all nodes that have a key with the specified name and a value that matches that of any node in the second argument. Otherwise, the returned node-set contains all nodes that have a key with the specified name and a value that matches the second argument’s string-value. Key names and values are assigned to nodes using the xsl:key element.