Axis child:: can be be omitted from a location step as it is the default axis. : Abbreivation « XPath « XML Tutorial

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2. XPath
3. Abbreivation

Axis attribute:: can be abbreviatet to @ 
. is short for self:: 

File: Data.xml
<?xml version="1.0" encoding="utf-8"?>
<data>
    <AAA id="a1" pos="start">
      <BBB id="b1"/>
      <BBB id="b2"/>
    </AAA>
    <AAA id="a2">
      <BBB id="b3"/>
      <BBB id="b4"/>
      <CCC id="c1">
        <CCC id="c2"/>
      </CCC>
      <BBB id="b5">
        <CCC id="c3"/>
      </BBB>
    </AAA>
</data>
File: Transform.xslt

<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet
      version="1.0"
      xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    <xsl:template match="AAA">
      <H3>
        <xsl:value-of select="name()"/>
        <xsl:text/>
        <xsl:value-of select="@id"/>
      </H3>
      <TABLE border="1">
        <TR>
          <td>full</TH>
          <td>abbreviated</TH>
        </TR>
        <TR>
          <TD>
            <xsl:text>child::BBB/attribute::id</xsl:text>
          </TD>
          <TD>
            <xsl:text>BBB/@id</xsl:text>
          </TD>
        </TR>
        <TR>
          <TD>
            <xsl:value-of select="child::BBB/attribute::id"/>
          </TD>
          <TD>
            <xsl:value-of select="BBB/@id"/>
          </TD>
        </TR>
      </TABLE>
    </xsl:template>
</xsl:stylesheet>

<?xml version="1.0" encoding="UTF-8"?>
    <H3>AAAa1</H3><TABLE border="1"><TR><td>full</TH><td>abbreviated</TH></TR><TR><TD>child::BBB/attribute::id</TD><TD>BBB/@id</TD></TR><TR><TD>b1</TD><TD>b1</TD></TR></TABLE>
    <H3>AAAa2</H3><TABLE border="1"><TR><td>full</TH><td>abbreviated</TH></TR><TR><TD>child::BBB/attribute::id</TD><TD>BBB/@id</TD></TR><TR><TD>b3</TD><TD>b3</TD></TR></TABLE>