Clarification on Staff Spacing

[Will Oram]

Reading old archives and documentation, I'm getting the idea that

        \property Staff.minimumVerticalExtent = #'(-12 . 12)

increases the space BETWEEN the staves. Instead, I see it increase the 
space on top and bottom of the system. It's a good tweak to know, but 
it's not what I'm looking for.

Given a piano staff, what property controls the distance between 
up/down? Or did I misposition the above property somehow?

Thanks.