Setting relative pitch as a global declaration?

[Alasdair McAndrew]

I'm sorry about all these damn-fool queries of mine; I promise to go back 
under my rock soon.  Anyway:
In the current 18th century suite I'm typesetting (for two treble 
instruments without bass), there is a separate variable (containing the 
notes) for each part of each movement.  Then there are global declarations 
about the instruments, and the key and time-signature of each movement; and 
these are all brought together in score blocks.

The one thing I don't know how to do is to declare the relative pitch 
globally.  Thus, each music variable looks like

movement1_part1 = \new Voice \relative c'' { notes, notes, and more notes }

The difficulty is that I want to re-set the second part for a bass 
instrument, so it might start off as

movement1_part2 = \new Voice \relative c { notes, notes, and more notes }

Currently this means changing the relative pitch for each movement 
individually.  It would be much more efficient to be able to do this just 
once at the beginning, with an appropriate global declaration.  Can this be 
done?  Is there a way to set the relative pitch of some music in a \global 
block?

(Note, I have indeed RTFM, but it's quite hard - even with the search 
function - to find answers to this, or examples of such use.  Hence this 
message...)

Thank you,
Alasdair
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