Choosing fins that will work best for you and your surfboard depends on your size, riding style, and your surfboard’s fin box.
For example, if you are in between size Medium and Large, you might pick size Medium fins for your small wave board, but size Large fins for you big wave board.
The two outer fins are closer to the middle of the board, angled towards the board center (”toed-in”) and can be flat on the inside in order to increase water tracking and speed.
Ideally, your board’s rocker matches the curve of the wave, which is why a more “rockered” board matches up well with a more curved, “hollow” wave.