\Paper syntax question

[Padmavyuha]

I'm struggling (as usual with new computer languages) with learning  
the syntax of the lilypond language.

I can't seem to find in the manuals or reference a clear explanation  
of how to set the left-margin value in a .ly file. The closest I got  
(or so I thought) was:

\paper { left-margin = 50\mm }

which is what the reference doc says, but it doesn't have any effect  
on the margin. There seem to be many different ways to trigger any  
particular variable, dependant on where you're putting it in the  
hierarchy, and I can't figure out how this one is supposed to work.

Help greatly appreciated...

- Padmavyuha