From BlenderWiki

[edit] Layout

• We have been working with a 2 column grid in order to lay out the buttons within a panel. The two columns of buttons can then be subdivided further horizontally (each column split into two, three, or more buttons) if needed. Columns can also be merged horizontally if necessary, to create layouts structured like this (for example):
  
  

-- JasonSaunders - 13 Dec 2004