title: How does jQuery work when there are multiple elements with the same ID value?
body: <p>I fetch data from Google's AdWords website which has multiple elements with the same <code>id</code>.</p>
  
  <p>Could you please explain why the following 3 queries doesn't result with the same answer (2)?</p>
  
  <p><a href="http://jsfiddle.net/P2j3f/1/">Live Demo</a></p>
  
  <p>HTML:</p>
  
  <pre><code>&lt;div&gt;
      &lt;span id="a"&gt;1&lt;/span&gt;
      &lt;span id="a"&gt;2&lt;/span&gt;
      &lt;span&gt;3&lt;/span&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>JS:</p>
  
  <pre><code>$(function() {
      var w = $("div");
      console.log($("#a").length);            // 1 - Why?
      console.log($("body #a").length);       // 2
      console.log($("#a", w).length);         // 2
  });
  </code></pre>