A payout system where the reel size stays the same, but the number of lines recognized as a win increases.

Expanding Paylines is a winning determination system where, typically upon entering a bonus round like free spins, the number of 'Paylines' recognized as a win increases while the physical size of the reels remains the same.

Its goal is to increase the winning probability by changing the 'rules' of what constitutes a win to be more favorable to the player, without altering the game's appearance.