DG>Spare set in case of a break, or second line.
 RS> Used for but not meant for?
Well, these days they're "meant for".  R/G is first line, Y/B is second line, 
and when wired up to a jack, a 2-line phone can be plugged in & both lines 
will be functional.
  >I think in the days of party
  >telephones they were connected, and had a different function.
 RS> Which was.
Not sure.  I've seen old phones where >2 wires were connected.  I assume it 
was some sort of ringer selector, so you could ring some phones but not 
others on the same line, but you really need someone who knows more then me 
about old-time telephones to answer this one.  This is all from the days of 
hard-wired telephones, before any kind of jacks.
Regards,
Dave
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