title: what are tb, rl and lr terms in CSS?
body: <p>I am reading <a href="http://www.w3.org/TR/css3-box/#collapsing-margins" rel="nofollow">CSS3 basic box model - collapsing margin</a> but I am unable to figure out <code>'tb', 'rl' and 'lr'</code>, terms.</p>
  
  <p>By name it looks like tob bottom for tb and right left for rl but what is the meaning of these terms in below paragraph?</p>
  
  <blockquote>
    <p>The top margin of a box A collapses with the top margin of its parent
    box B if the margins are adjoining and B is ‘tb’.</p>
    
    <p>Note that A itself may be vertical box.</p>
    
    <p>The bottom margin of a box A collapses with the bottom margin of its
     parent box B if the margins are adjoining and B is ‘tb’.</p>
    
    <p>The left margin of a box A collapses with the left margin of its
    parent box B if the margins are adjoining and B is ‘rl’ or ‘lr’.</p>
    
    <p>The right margin of a box A collapses with the right margin of its
    parent box B if the margins are adjoining and B is ‘rl’ or ‘lr’.</p>
    
    <p>The top margin of a box A collapses with the bottom margin of a
    sibling box B if the margins are adjoining and their parent is ‘tb’.</p>
    
    <p>A right margin of a box A collapses with the left margin of a sibling
    box B if the margins are adjoining and their parent is ‘rl’ or ‘lr’.</p>
  </blockquote>