Pick ups and phase issues?

[Venom of God]

Could someone explain this to me please? I've heard a bit about phase problems when running two pick ups, but don't really understand it.

[line6man]

When you have two pickups out of phase, it means that one pickup's signal is on it's positive cycles while the other pickup's signal is on it's negative cycles, so combining the two signals cancels them out to various degrees.

Destructive and constructive interference are explained in detail here: http://en.wikipedia.org/wiki/Destructive_interference