Can someone explain what the third expression in python's indexing syntax represents?
<blockquote>
  <p><strong>Possible Duplicate:</strong><br>
  <a href="http://stackoverflow.com/questions/509211/good-primer-for-python-slice-notation">Good Primer for Python Slice Notation</a>  </p>
</blockquote>



<p>I first ran across it last night for reversing a string <code>"Hello"[::-1]</code> and I can't figure out how it actually works.</p>

<pre><code>"Hello"[::-1] # returns "olleH"
[1,2,3,4,5][::-1] # returns [5,4,3,2,1]
"Hello"[1:5:1] # returns "ello"
"Hello"[1:5:2] # returns "el"
</code></pre>

<p>My searches for "third expression in python index" have come up empty. What is this expression and how does it work. Obviously ::-1 reverses the list but I can't figure out what the other, positive values represent.</p>
