In the conventional world of bricks and mortar, the physical limits on shelf space determine the volume and variety of goods that companies can display and sell at any one time. In every retail venue, the battle is all about who gets to use the most attractive, best-located shelf space. In the real world of grocery stores and other retail spaces, shelf space is finite -- even in the largest Wal-Marts. Only the most powerful vendors get the best space, and their power is defined ultimately by their ability to pull sales through the shelve

But shelf space also exists outside the physical world of supermarkets and shopping malls. Take the world of network TV, where time-slots for programming are the equivalent of shelves. In that world, the most desirable shelves are those that more viewers "visit." So “buying time” for an ad during prime time on a highly rated show is far more costly than buying time for the same ad at odd hours, not to mention at 3 a.m., when ratings are lowest. Likewise, even the broadcast networks have lost their dominance, they still have relatively high traffic; in that way, shelf space on ABC or CBS is more expensive than the equivalent times on the Golf Channel or Bravo.