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.row defines a row of divs, like the name implies. Each one indicates a new line of divs, no matter if the above line is full or not

<div class="container">
  <div class="row">
    <!-- These divs are inline and do NOT fill up the full 12 columns -->
    <div class="span4">...</div>
    <div class="span4">...</div>
  </div>

  <!-- This is a automatically a new line of divs -->
  <div class="row">
    <!-- This div will appear below the previous row, even though it
      would fit next to the other divs -->
    <div class="span4"></div>
  </div>

  <!-- These will appear in their own row, but may act
    unexpectedly in certain situations -->
  <div class="span4"></div>
  <div class="span4"></div>
</div>