Add-A-Ball —
A feature designed to provide a reward to the player in regions where replays (free games) were outlawed as a thing of value, making pinball into gambling. Add-A-Ball games allow the player to be awarded multiple additional balls, and usually include a counter showing balls remaining to play ("Balls to Play" rather than the usual "Ball in Play" ) that is incremented as each ball is awarded. Note that this is different from the feature of awarding an extra ball (common to many games) because those games do not change the ball counter when the extra ball is played, and often can only award a single outstanding extra ball at any given time. Therefore, a machine is a true Add-a-ball machine only if you can earn more than one extra ball per ball in play.  Most machines are a replay type where a special scores a free credit and only one extra ball can be awarded per ball in play.
Add-a-ball games start a ball counter at the original number of balls given to a player at the start of the game and then award the player additional balls as objectives are met, incrementing the "current ball" count for each one.

Early add-a-ball games had individually lighted numbers on the backglass indicating the current ball count.  These were usually completely separate models from the non-Add-a-ball games (even of the same name), but in later years an Add-A-Ball "Option" was instead built into a single model, allowing use as either an Add-A-Ball game, a replay game, or a novelty play game, depending of what the government rules in effect at the location.

Because some locations even disallowed the display of the replay or add-a-ball totals wheel, some games came with a sticker to cover the replay counter.  Some games would even "hide" the extra ball count in backglass graphics that could not be seen until an additional ball was added.

Gottlieb games can usually award up to 5 additional balls at any one time.  If one or more of these additional balls are awarded, they are all played before the main ball counter decrements.  So you could win up to 5 balls during the play of 5 regular balls, or a total of up to 25 extra balls. Gottlieb called this the "WOW" feature.  For Williams games, the ball counter started at 5 balls and awarded balls could raise the total to as 10 balls at one time.

Alvin Gottlieb created the concept which was then designed by Wayne Neyens.