In the beginning, all slot machines were single-line machines with three reels. You only win if you matched the symbols in a horizontal line across the three reels. Now, most slots have 5 reels and multiple paylines. Players have the ability of choosing how many paylines they want to put in play. The more paylines you play, the better your odds of winning. The catch is that it costs you more money per spin, since you pay per line.