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The Theme Song and the Soundtrack Albums
The theme song is called "I'll Be There For You." It is sung by the Rembrandts. The music is by Michael Skloff and the lyrics by Allee Willis.
The Soundtrack album has the TV version and the extended version of the song.
The extended version can also be found on the Rembrandts 1995 album, "L.P."
And a new version can be found on the 2nd Friends Soundtrack Album, Friends Again.

And to answer the most-asked questions about the theme song: There are only four claps; and D.O.A. stands for "dead on arrival."

Lyrics:

So no one told you life was gonna be this way [four claps]
Your job's a joke, you're broke, your love life's D.O.A.
It's like you're always stuck in second gear
When it hasn't been your day, your week, your month, or even your year

CHORUS:
but I'll be there for you
(When the rain starts to pour)
I'll be there for you
(Like I've been there before)
I'll be there for you
('Cause you're there for me too)

Then, the extended version continues with:

You're still in bed at ten and work began at eight
You've burned your breakfast; so far, things are going great
Your mother warned you there'd be days like these
But she didn't tell when the world has brought you down to your knees

CHORUS

BRIDGE:
No one could ever know me, no one could ever see me
Seems you're the only one who knows what it's like to be me
Someone to face the day with, make it through all the rest with
Someone I'll always laugh with
Even at my worst, I'm best with you
Yeah!

It's like you're always stuck in second gear
When it hasn't been your day, your week, your month, or even your year, but

CHORUS