The word payline is the term used to refer to the line that the reels on a slot machine must stop on to trigger a win. The majority of slot machines will have a single payline. But 3 and 5 line are also common among mechanical slots. While video slots commonly have 5, 9, 15 or more lines. With 9 being the most common. Each payline requires its own separate coin. So it can be rather expensive to play a 15 line machine.