This article is from the Friends FAQ, by Andy J. Williams AndyJW@dartmouth.edu with numerous contributions by others.
JG: "O.K., here's the quasi-definitive answer as I know it, as we writer
folk have discussed it over the course of the season...
"In the pilot, we established that Monica, Rachel and Ross all went to
Lincoln High School together. Monica was fat; Rachel was popular and
possibly bitchy; Ross was a nerd. Rachel says that she and Monica were
close, but in the time since, they've drifted -- enough so that Rachel
didn't think to invite Monica to her wedding.
"We also established that Ross had a crush on Rachel back then; she knew
this, but thought of him as 'Monica's geeky older brother.'
"During the making of 'The One Where Nana Dies Twice' [108], Schwimmer and
Perry wondered if Chandler and Ross might have gone to college together. We
writers agreed that it might be nice for two of our other characters to have
a history together, so we wrote it in.
"Therefore, my guess is that when Monica graduated college and was looking
for an apartment, Chandler (who was living with his first roommate Kip --
the one Joey is jealous of in 'Dozen Lasagnas' [112]) alerted her to the
fact that an apartment was opening up across the hall from him. Monica found
Phoebe through a roommate-matching service and moved in.
"One of the ways we've justified -- to ourselves, at least -- this amazing
place Monica lives in (on an assistant chef's salary!) is that it's probably
an illegal sublet. Several plot lines revolving around this fact were
discussed over the course of the season, but never amounted to anything."
(Alexa later added: "Rachel and Monica live in a rent-controlled
apartment Monica's grandmother left her (not Nana - the other one)
This information was actually in an episode, but cut out for
time.")
JG: "Anywaaaaaay... When Kip got married, he moved out and Joey (who
Chandler probably also met through a roommate-matching service, or by
putting up flyers at the laundromat) moved in. Around the same time,
Phoebe's grandmother needed some kind of taking care of, so Phoebe moved out
of Monica's to live with her. This is another element we talked about a lot
over the course of the season, but it never found its way into a story.
"As for the timing of the events surrounding Ross's divorce, that's
something we've consciously avoided getting into. The more you think about
it, the more troublesome it becomes: Was Carol already involved with Susan
when Ross got her pregnant? We invite viewers to figure this one out for
themselves..."
 
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