generally it's recommended that humbuckers use 500k post... single coil pickups use 250k pots... and active pickups use 25k pots. there are other pots that can be used... it depends on what tone you want... you CAN use 500k and 250k on either humbuckers or single coils... 500k pots tend to bleed off more treble and 250k pots tend to sharpen tone...