Suppose I have state.xml and address.xml. state.xml has the state name, abbreviation, and its capital city. address.xml has person's name, street address, city, state, zip code. When I open address.xml and go to address that has new york in state, I want to extend address.xml to reference state.xml and display the new york abbreviation and if i go to an address with new jersey, I want it is abbreviation and...so now and so now. How do I do that?

Something like this. Use address.xml as the principal input document. Do

<xsl:variable name="addresses" select="/"/>
<xsl:variable name="states" select="document('state.xml')"/>

<xsl:template match="address">
  <address>
    <xsl:copy-of select="*"/>
    <xsl:copy-of select="$states/states/state[name=current()/state]/abbreviation"/>
  </address>
</xsl:template>

If the look-up table is large, you can use keys (xsl:key) for greater efficiency.

Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference

thanks