scheme numeric values

[Graham Percival]

Hi all,

As far as I understand, most numbers in lilypond tweaks are measured in 
staff-spaces.  For example,

\override TextScript #'padding = #2.5
will place TextScript objects 2.5 staff-spaces away from the note they 
are attached to.

If that's correct, then what's happening here?  (example from Appendix 
B, scheme tutorial)
-----
This syntax will be used very frequently, since many of the layout 
tweaks involve assigning (Scheme) values to internal variables, for 
example

\override Stem #'thickness = #2.6

This instruction adjusts the appearance of stems. The value 2.6 is put 
into the thickness variable of a Stem object. This makes stems almost 
twice as thick as their normal size.
-----

The thickness of the stem is obviously not 2.6 staff-spaces, so what 
*is* it measured in?  And how do users know whether a number is being 
measured in staff-spaces or other stuff?

Cheers,
- Graham