title: Checkerboard Pattern css selectors
body: <p>I have a list of div elements that I'm currently displaying in four columns using CSS float. I would like to "alternate" the background-color of these elements. If I simply use nth-child(even) or nth-child(odd) I get the same color in each column vertically. But i want it like this:</p>
  
  <pre><code>1blue  2green 3blue  4green
  5green 6blue  7green 8blue
  9blue ...
  </code></pre>
  
  <p>I really don't get it. Tried 4n+1 and so on, nothing works as expected.</p>