Every pickup coil has two properties that affect how they will sound when they are combined with others. These properties are called phase and polarity. Phase is the direction current travels through the pickup, and polarity is the direction of the magnetic field. With both properties, there are only two options. Phase can either be “top coming” or “top going”, and polarity can either be “south” or “north.”
A typical application of these principles is with single coil pickups. One interesting property of single coils is that depending on their phase and polarity, it may be possible for them to be hum cancelling when combined. Modern Fender guitars are typically wired in such a way. The key to a strong, full tone with hum cancelling is to combine single coil pickups that have both opposite phase and opposite polarity. If one pickup is north polarity, top going, the other must be south polarity, top coming. This is why most Strat middle pickups and Tele neck pickups are what is known as reverse wind, reverse polarity, or RWRP for short. With a RWRP middle pickup in a Strat, for example, you will get hum cancelling in positions 2 and 4 on a 5-way switch.
Humbuckers all operate on this principle. The two coils have opposite wind and opposite polarity. When combined, you get a fat, meaty tone and no hum. Splitting a humbucker will, of course, bring back the hum.
So, what happens when single coil pickups are mismatched? You will either get hum or phase cancellation. We all know what hum is. Phase cancellation is what happens when two pickups interfere with each other’s frequency responses, and most people find the result to be “thin” or “hollow” sounding. When this happens, we say that the pickups are out of phase. It’s a tone you might like in certain situations, and some great players like Peter Green use an out-of-phase tone by choice. Most of the time, though, it is something we try to avoid.
For two single coil pickups to be in phase, both the magnet polarity and the wind direction have to either be identical, or opposite. In other words, two pickups with the same wind and polarity will be in phase, and so will two pickups that have opposite polarity and wind. If the two pickups have the same wind but different polarity, or the same polarity but different wind, they will be out of phase with each other.
The most common reason for two single coils to be out of phase is that one of them is wired backwards. The lead that was supposed to be connected to ground is connected to the output, and vice versa. If you get phase cancellation when combining two single coils, the easiest cure is to swap the lead wires on one of them (not both, or you will just have the same problem).
Bottom line: if you want two coils to be in phase and hum cancelling, you will need one of them – and only one of them – to be RWRP. If you’re using pickups from the same manufacturer it’s pretty straightforward to match wind and polarity. They are all designed to work together. A Seymour Duncan single coil pairs perfectly with a Seymour Duncan RWRP single coil. Unfortunately, this is not true if you mix and match pickups from different manufacturers. One manufacturer’s idea of a standard single coil might be another’s idea of a RWRP, and vice versa.
The most common scenario in which the above comes to bear is when combining Seymour Duncan single coils and Fender single coils. It all started when Seymour decided to base his single coils on the original Fender single coils from the 1950′s, which were south, top going. At some point in the 60′s, Fender decided to change their single coils to be north, top coming. The end result is that Seymour’s regular pickups are equivalent to Fender’s RWRP, and vice versa. The only exception to this is the Antiquity Texas Hot; that’s because it’s an accurate re-creation of the “new” style of Fender pickup.
This isn’t really a big deal, as long as you’re prepared for it. Let’s say, for example, you are replacing the bridge pickup in your Fender Strat with a Duncan SSL-3T, but you’ve decided you want to keep the stock Fender middle and neck pickups. Since you want your notch position tone to be hum canceling and in phase, you will want to make sure your SSL-3T has the opposite wind and polarity of your Fender middle pickup. This means you will be needing an RWRP SSL-3T. It might seem weird to order an RWRP pickup for the bridge, but if you want it to hum cancel with that Fender single coil, that’s what you’ll have to do.