What happens when two pickups are out of phase?

When two pickups are out of phase with each other, there is significant signal cancellation across the guitar’s entire frequency range. This results in a hollow sound and a drop in output. However, two pickups are never either completely in or out of phase, because they are located under different parts of the string, and are therefore cancelling different frequencies. The specific distance between the 2 pickups and the specific pickup models will affect how much cancellation occurs. The out-of-phase output drop is most apparent with a dead-clean amp setting. Overdriven amps tend to compress, which levels out differences in output, so the drop isn’t as obvious.