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Two pickups can be combined in 4 different way. They can be parallel or series, either in or out of phase:
With the two pickups each on their own, that's a total of 6 different sounds for the price of just 2 pickups. If you make coil taps available, or add another pickup, the combinations can very quickly become unmanageable, particularly if you play live.
The usual way to combine pickups is in parallel and in phase. In fact, this may be the only option you have for pickups that are sealed and wired internally, providing only a shield and "hot" wire for connection.
Either way, a properly matched pair of pickups wired in parallel and in phase gives a great country-rock or jazz-rock sound. The strat pickup combinations give a funky sound which oozes character (after all, if it's good enough for Mark Knopfler and Robert Cray, it's good enough for me!) This strat sound is often mistakenly called an out of phase sound, because its slightly nasal character is typical of out of phase sounds.
To provide a sensible number of alternatives requires a little time and experimentation. I can offer some recommendations, but the ideal way to find the good pickup combinations is to first install the pickups where you like their individual sounds, then bring all of the pickup wires outside the guitar and try different wiring arrangements.
If you're not sure how to go about this, ask a friend with some electronics knowledge for help. After you have found a realistic number of good sounds, try to come up with a practical switching arrangement. My rule of thumb is that you should never need to move more than 2 switches to get to any sound, and your main solo sound(s) should be accessible from just one master switch.
If you play live, you may find it useful to experiment by playing along to music, with the music and guitar set to similar volume levels. This is because sounds that impress you in the peace and quiet of your own practice room (or recorded) may not have enough highs or middle punch to cut clearly through a band. The music will help mask these "great but useless" sounds.
An easier path is to stick to a few traditional set-ups, which are guaranteed to give good results as long as you follow a few simple rules.
The different ways to wire 2 pickups:
If you're not interested in the technicalities of how humbuckers work, skip this section and go straight to ...
Now that we've seen different ways to connect pickup coils, it's a good time to take a quick detour to consider how humbucking pickups work. Hum-bucking means cancelling hum as well as other electromagnetic interference that surrounds us. Hum and interference radiates from many common sources found in musical environments including mains transformers, PC monitors, lighting dimmers and switching systems.
A humbucking pickup has 2 coils side by side. Each of these coils picks up hum and interference as well as guitar string vibrations. This picture of humbucking pickups shows the 2 coils clearly in the models without covers. Even though covers enclose one of the coils, it still picks up the strings fine.
You would think that if coils can be combined to cancel hum, they should also cancel the guitar signal. The diagram above shows in phase and out of phase connections. Signals that are in phase with each other add, while those out of phase cancel. It doesn't matter if the two pickups are connected in series or parallel.
For a humbucking pickup to work, the coils need to be out of phase for interference, but in phase for the guitar signal. There are two things that affect the phase of both interference and the guitar signal:
1. The coil winding direction
The guitar signal is caused by strings vibrating in the coil's magnetic field. A third thing affects the phase of the guitar signal only:
3. The magnetic polarity
Changing any one of these 3 items will reverse the phase of the pickup. Changing any two of these items does nothing, the phase is reversed twice, so it's back in phase again! Changing all three reverses the phase, because it's been reversed 3 times.
For example, combining 2 single coil pickups that are wound in opposite directions will cancel hum. The two coils inside a humbucking pickup have opposite coil connections or opposite coil winding direction, it doesn't matter which. They also have opposite magnetic polarity, so they're out of phase for hum and interference, but in phase for the guitar signal.
Recent strat guitars are commonly sold with the middle pickup RWRP (reverse wound, reverse polarity), so even though the pickups hum on their own, hum will be cancelled when the middle pickup is combined with either the neck or bridge pickup in the "in-between" selector positions. If the pickups are identical and close to each other, they'll cancel nearly all hum, however, different pickups and/or pickups placed further apart will have less cancellation.
Before we look at switching options and pickup choices, we need to decide what we want from our guitar sounds.
Some players simply want to switch between a low-level rhythm sound and a high level solo sound with your pickup switch. If that's you, then you'll be served well by a guitar with pickup types and heights set to suit that option. The two pickups are unlikely to mix together well, but if you don't use that sound, it doesn't matter anyway. Surprisingly, if you mix a weak pickup with a strong pickup (as usual, in parallel and in phase), the weaker pickup will usually dominate - see below for more. A common setup for solo switching is a hot humbucker with a vintage neck humbucker (often adjusted to a low height), or a neck and maybe middle single coil. Another option is a regular neck humbucker for 2 solo sounds and a middle single coil for rhythm. You may have seen abbreviations HSS for humbucker-single-single and HSH for humbucker-single-humbucker. SSS is a guitar with 3 single coils, such as a strat.
I recently played a fantastic quality Ibanez with switching options between HSH and SSS and I was really impressed with the humbucker tones, but surprised at the massive volume drop with single-coil sounds. The drop was much more than a normal humbucker coil-tap. This will suit players wanting to use pickup selections for rhythm/solo switching. It's also a good option for players who use a lot of overdrive: the solo sounds will be strong, while the rhythm sounds will be clearer and cleaner.
If simple solo switching is all you want from your guitar, then you don't have to worry about the matching traps and fancy combinations described below. Just choose the pickup you want for each position, and play.
Players (like me) who use other means to set rhythm and solo levels, such as boosters or volume pedals, will be served better by pickup selections that are all roughly the same level. The advantage of this approach is that you have several sounds available for both rhythm and soloing. Those sounds include pickups on their own as well as pickup combinations such as those drawn above. However, it can be tricky finding pickups that work well together.
Vintage guitars all used the same pickup in all positions (it wasn't until the '70s that players started using replacement pickups and experimenting with different pickup mixes). So a vintage strat or les Paul has the same pickup in all positions and it should be no surprise that these mix with each other perfectly electronically. Usually the bridge pickup is placed as high as possible without string vibrations touching it, then the neck pickup is lowered for an even volume balance. These pickup combinations are guaranteed to be a good mix when used together.
Identical pickups can make it hard to find amp tone settings that avoid boomy bass on the neck pickup and piercing treble on the bridge pickup. Often tones are set for a compromise, which really means you're not getting ideal tones from either pickup.
A better approach is to look for pickups in each position that have a similar tone balance while still allowing the natural timbre to shine through. This usually means a hotter bridge pickup with more mids and bass and a vintage neck pickup with more highs. That way, you don't have to compromise anything with your amp tone settings and you have a wide selection of different guitar sounds.
Probably the most common trap is to go for a super-hot bridge pickup with a clean neck pickup. A great idea in theory, and fine if they are the only two sounds you want, but when you combine these pickups (in the usual way - in parallel and in phase), it doesn't sound very different from the neck pickup on its own.
You might have expected the hot bridge pickup to dominate, but the opposite happens! It's caused by an impedance mismatch between the pickups, and the lower impedance single coil shunts (or drains) much of the sound of the higher impedance humbucking pickup. The lower impedance neck pickup, however, is hardly affected by the higher impedance bridge pickup.
You can avoid this trap by reading manufacturers' websites, or emailing for advice on ideal pickup mixes. DiMarzio publish millivolts for their pickups which makes it easy to select: as long as bridge pickup is the same or a little more than the neck, they'll mix well.
Some players make a fuss about DC resistance, but it's really not much of a guide to the pickup's mixing suitability; it can even be quite misleading if the pickup uses dummy hum-cancelling coils. But if you have no other way to find out, you can measure DC resistance with a multimeter. You can expect to measure values of about 6 Kohms for lower impedance single coil models, 7-8K for vintage humbuckers, up to around 15 Kohms or more for the higher impedance super-hot humbuckers. If one pickup is over double the resistance of the other, you can be fairly sure they won't mix well. Less than double will at least provide a noticeable difference, but of course identical pickups are guaranteed to combine best.
If your guitar provides a separate volume control for each pickup, you can compensate for mismatched pickups, but I find this fiddly, and not as clear as a properly matched pickup combination. For this reason I prefer to have the pickups and switching come before a master volume control, and use the master volume control for level changes.
Combining pickups in parallel and in phase provides a special tone due largely to the inductive (coil) load each pickup places on the other, as well as the way they reinforce and cancel each other out at certain note harmonics. This sound is not the same as combining pickups electronically, such as using a separate pre-amp channel for each pickup, or combinations found in some active pre-amp guitars.
Not all pickups have the same "polarity", even similar models from the same manufacturer are not guaranteed to be the same. Polarity refers to the phase of the signal in relation to the string vibration. Most custom pickups provide (as indeed ALL pickups should) a pair of wires for each coil, and a separate shield connection which must be earthed. At the other end of the scale you will find some pickups with their connections encapsulated in epoxy resin, providing only a single wire and shield.
Reversing the coil connections reverses the polarity, and may be necessary on one pickup to allow the standard "in-phase" connections. Your ears are a good guide - if you expect an in-phase sound you should hear a full, slightly nasal sound. If you get a hollow, middley sound with a volume drop, you probably have pickups with opposite polarity, and need to reverse the connections on one pickup to restore normality.
Combining pickups in parallel, but out of phase is less sensitive to impedance differences, and this may the best way of getting some useful variety if you're stuck with a guitar with mismatched pickups.
Combining pickups in series is a matter of personal taste and experimentation, but I recommend NOT using similar sounding pickups. In phase it's just louder and mushier, while out of phase you will find a dramatic loss of volume. On the other hand, wiring very different sounding pickups in series will add their sounds, and exaggerate their differences when wired out of phase.
There are plenty of wiring ideas on my Pickup Wiring page. These start with standard wiring schematics and common mods, then show some advanced and downright sneaky mods!