Whats the Difference????

There is rails,one large one per string like on musicmans, two poles on above the string and one below like on jazz basses and one below and to the left and one abve and to the right like here

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[walterw]

you forgot one little pole under each string (sting bass/original p-bass), one pole between each string (roscoe beck), a row of little screws right next to each other (carvin, q-tuner), etc., etc.

each variation sounds different, and each is part of a differently designed pickup.

in general, a pole under each string gives more attack, while the "two poles just outside each string" design reduces that initial spike a little.

in general, poles that are themselves magnets (vintage-style fender and MM stingray) will have more attack but less output than steel slugs with the magnets underneath (mexican fender pickups, anything with adjustable screws for poles).