Case Sensitivity : Well Formed « Introduction « XML Tutorial

1. XML Tutorial
2. Introduction
3. Well Formed

<first> is different from <FIRST>, which is different from <First>. 

It's a good idea to pick a naming style and stick to it. 
Some examples of common styles are as follows: 

<first_name> 
<firstName> 
<first-name> 
<FirstName>