dabac is correct about the width of the chain/link. The more gears the tighter/less space the rear cassette has. You can use a higher number chain/link but not a smaller number. If you have say a 9 spd cassette you can use a 9,10 or 11 spd link. If you used a 8 spd link you would likely have wear and binding problems each time it went through the cassette.