title: What does the question mark in "Decimal?" mean?
body: <p>I am following tutorials about MVC4 asp.net. The tutorials are using <code>Decimal</code> like this:</p>
  
  <pre><code>     public Decimal x{ set; get; }
       public Decimal? y{ set; get; }
  </code></pre>
  
  <p>what does it mean when using <code>?</code>  after <code>Decimal</code></p>