The 1988 book Pinball The Lure of the Silver Ball stated that a "split bank" was invented by Norm Clark for the 2-player Williams' 1966 '8 Ball' and reused again on the 2-player 'Solids N Stripes', the purpose being to individually track on the backglass the billiard balls earned by each player, by lighting them, and to allow these earned billiard balls to carry-over from ball to ball for each player instead of resetting at the start of every ball. However, '8 Ball' did not use relay banks for this function, instead using dual stepper units. 'Solids N Stripes' is the first game to employ Clark's "split bank" method of ball-to-ball carry-over for multi-player games, actually just two separate relay banks, each reset by its own reset coil.
One bank is for billiard balls 1 to 7 (first player) and the other bank is for 9 to 15 (second player). The Player Relay selects to be active only the bank associated for the player being served and thus "splits" the duty cycle of these banks. For one-player operation, the second bank remains inactive for the entire play.
The billiard balls do not have to be earned in numerical sequence. When a player earns all seven billiard balls for that player, the 8-ball pocket lights to award an extra ball for that player. When an extra ball is played, only then does the relay bank for that player reset, turning off the seven lighted billiard balls to allow the player to earn them again.
On '8 Ball', the billiard balls have to be earned in numerical sequence so it made sense for that game to use for each player a sequential mechanism such as a stepper unit with reset coil. Because the billiard balls on 'Solids N Stripes' can be earned in any random order, it makes sense to use a relay bank which will track them in any order in which they are earned. The two relay banks were mounted under the upper portion of the playfield and, with their associated relays, reportedly made the playfield unusually heavy to remove from the game.
