^   t                                                                         
  |   t     t                                                           t       
  |   t  t  t                                                           t       
  |   t  t  t                                               t  t  t  t  t  t    
  |   t  t  t  t                                t  t  t  t  t  t  t  t  t  t    
  |   t  t  t  t                                t  t  t  t  t  t  t  t  t  t    
  |   t  t  t  t                                t  t  t  t  t  t  t  t  t  t    
  |   t  t  t  t  t  t                 t     t  t  t  t  t  t  t  t  t  t  t    
  |   t  t  t  t  t  t        t        t  t  t  t  t  t  t  t  t  t  t  t  t    
  |   t  t  t  t  t rt rt  t rt  t  t rt  t  t  t  t  t rt  t  t  t  t  t rt    
 -+---------------------------------------------------------------------------> 
  |  19 20 21 22 23 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18