out of phase

simply put, if you take a couple pu's and wire them into a guitar so
that they're out of phase, they sound exactly as they do normally
untill you use them together. then you''ll notice a big difference. for
example, if the bridge and middle are out of phase with each other,
you'll get a thin weak sound when you use them both at the same
time, but seperatly they'll sound normal. to fix it, simply reverse the
hot and ground wires of one of the pu's. however, if that happened
in a 3 pu guitar, you'd first want to see if the neck pu is out of phase
with the middle. if so, then what you have is the bridge and neck
out of phase with the middle. since the middle is the odd man out,
reverse it's wires.if the neck is in phase with the middle, then the
bridge is the odd man out, and it's wires should be reversed.

theres a trick u can use to see if they're in phase before you put the
guitar back together. put the red lead of a ohm meter to the hot of
one of the pu's, and the black to ground. then take something
magnetic like a screwdriver and slowly move it from about 6"
above the polepieces down towards them. keep your eye on the
meter to see if it goes up or down. then check the other pu's. they
should make the meter go the same way. if not, reverse the wires of
the one that doesn't read the same. make sure they all read the
same way and you can be sure they're all in phase before you put
the axe back together. it can save you a lot of headaches.

also, if there are humbuckers in the guitar,you'll want those to be
wired up with the hot wire to the switch as per the manufacturer.
don't reverse it. instead, reverse the single coil[s]. this is because if
you reverse the wires on a bucker, the chassis becomes hot and
acts as a noise antenna. single coils don't have that problem since
they don't have metal chassis.
if it's a guitar with 2 or 3 humbuckers that are out of phase when
wired up as per the manufacturer, the one will just have to be
reversed anyway. it'll hum when you touch the mounting screws,
and generally be noisier, but it's either that or have an out of phase
sound when using them together.

by the way, this phase problem will only happen when mixing pu's
from different manufacturers.

dale 

To add to Dale's very thorough reply...

ALL pickups are ALWAYS out of phase with other pickups, even
if only to a limited degree, simply because they cannot occupy the
same spot on the string.

Huh?

When two pickups detect the same string vibration, but the output
of one pickup is electrically out of phase (180 degrees opposite)
with the other, their combined outputs electronically cancel each
other. 

As a physical body, the vibrational amplitude of the fundamental
and different harmonics happens to be different along the string,
which is why the same pickup near the bridge on the same guitar
yields a different tone than the same pickup near the neck. Not only
are the relative intensities of those harmonics likely to be different,
but the phase relationship of the same harmonics at different points
along the string can also be different, moreso if those pickups are
close to each other. I'm sure if you took the appropriate strobe
picture of a string in mid-twang, you could see that - at some
frequencies - the string would be close to the polepieces at one
pickup, but away from the polepieces at the other pickup. That's
how they wiggle, and this is why combining pickups almost always
gives you something very different than either one alone: because
they not only ADD, but SUBTRACT too.

Case in point is the neck/middle or middle/bridge combo on a
Strat. Tell me either one of those bears a close resemblance to
either of the pickups involved. They don't because they don't
simply add; there is also phase-related cancellation of harmonics
because the two pickups are sensing sorta the same content...but
not quite. You will get this whether you have a
reverse-wound/reverse-polarity middle pickup or not (phase
cancellation of hum is another matter).

Why am I telling you this? To help identify conditions under which
electronically phase-reversed pickups yield different sorts of
outcomes.

If two pickups detect the exact same "stuff" and are phase
reversed, they will completely cancel out. As the overlap in their
individual spectral content or amplitude decreases, the amount of
apparent cancellation will decrease, and the combined volume will
start to approximate the output of each pickup individually. If you
have a guitar with phase-reversible pickups, and individual volume
controls, you will find that simply edging down the volume of one
pickup will magically raise their combined level. If the pickups are
not precisely matched, you may find that edging down the volume
of the louder one will result in a sudden drop in combined level
(where maximal cancellation occurs), followed by a resumption of
higher output as you edge the control down even more (so their
individual levels are not matched). Better matching, not only for
content, but for level, gets more cancellation.

The *electronic* reversal of phase simply increases the likelihood
of more cancellation taking place; when, and if, the pickups
involved are more matched for output and spectral content.

Depending on the tonal characteristics of the pickups one has on
one's guitar (and it is increasingly rare to find the identical pickup at
different points along the string in commercial products), you may
find that phase reversal can yield anything from no perceptible
difference, to a different nuance which may disappear unless played
through a pristine clean signal chain, to an objectionable honkiness
or drop in output. You may also find that simply having separate
pickup volume controls without phase-reversal *wiring* can offer
up an interesting palette of naturally occurring (i.e., not wiring
related) cancellation.