There are always two signals in any given "sign."
The first says,"listen up!
I'm gonna say something!!"
The second tells what to do.
The first is always the same.
The second is the variable.
In some cases
(asshole in the front row)

the second actually leads
to a third,
but this is usually not the case.
The first signal is a high pitched trill,
usually carried for twomeasures.
Anyone--Trey, Mike or Page may do it
(But not Fish:
"Fish talks so much in real life that we don't let him talk here!"--Trey).

If the jam is really hectic, the signal is different.
It is a one measure figure which sounds like CCCC/Bb Bb/G G/Bb
(slash = beat division),
although it changes with different keys.
This used to be the signal
(listen to old bootlegs),
but is usedonly rarely now.

The second signal:
here is a catalogue of signals based on Trey'sintro,
and on my own observations:

1) Simpsons theme song (you know!):
Everyone goes "D'OH!!"

2) Music to "To everything, turn, turn, turn":
Everyone turns around and screams.

3) Four falling notes:
a one measure figure, each note is bent down to the next
(on the guitar, anyway):
everyone falls down.

4) Triplets of a strange sounding altered chord on beats 1,3,1,3
(two measures):
point to asshole in the front row.
(What do they do after this?
Anyone at Portsmouth that can explain the cheering?)

5) One measure figure that sounds like zip zip zip zip!
(i.e. hand slides up the fretboard very quickly 4x):
Band immediately plays a four bar waltz stomp.

6) music to "it don't mean a thing if it ain't got that swing):
band cuts immediately to swing beat.

7) scraping sound on the guitar
(similar to #5, except that scrapes are lower pitched, and go both down and up the fretboard):
everyone shouts, "AWWW FUCK!"

8) Circus theme
(listen to the intro to esther--but note!
This is not a signal!
It lacks the first part--the attention getter.):
everyone sings a random note.

That's all I have been able to decipher up to this point,
although I suspect the presence of at least two more:
One that just says,stop and freeze!
One that says, stop and wave your hands up anddown wildly
(the strobes come on for this one).