Comments start with the string : Comments « Introduction « XML Tutorial

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2. Introduction
3. Comments

<name nickname='007'> 
    <first>James</first> 
    <!--James does not have middle name --> 
    <middle></middle> 
    <last>Bond</last> 
</name> 

You can't have a comment inside a tag, so the following is illegal: 

<name nickname='007'> 
    <first>James</first> 
    <middle></middle <!--James does not have middle name -->  > 
    <last>Bond</last> 
</name> 

You can't use the double-dash string (--)inside a comment: 

<name nickname='007'> 
    <first>James</first> 
    <middle></middle <!--James does -- not have middle name -->  > 
    <last>Bond</last> 
</name>