Pickups In/Out of Phase
Whether 2 coils are wired in series or parallel affects both tone and output. The animations below suggest a 2-coil humbucking pickup, but these principals also apply to two single-coil pickups.
Animation 1 shows current flowing through 2 coils wired in series.
Notice that current must flow through one coil before it flows through
the second. This wiring, common in standard humbucking pickups, increases
output and decreases treble response.
Animation 2 shows current flowing through 2 coils simultaneously.
These coils are wired in parallel. Parallel
wiring retains much of each coils' "single coil" tone, but has less output
than series wiring. In the case of a humbucking pickup, this parallel
wiring is still noise-free or "humbucking". Parallel wiring is common
for guitars with single coil pickups when more than one pickup is active.