<xsl:key>

The <xsl:key> element declares a key for subsequent use with the key() function.

<xsl:key 
  name = qname 
  match = pattern 
  use = expression /> 
name

The name of this key

match

Selects the nodes which are in this key set

use

Selects the expression within each node that identifies the key

The <xsl:key> element is only useful in conjunction with the key function:

node-set key(name, object)

Returns the nodes that match object in the specified key