Channel Islands CI Mid fibreglass 2+1 fin set is consists of a 6.5 inch centre fin and side bites. There are four side bite options to choose in the drop down menu. 

Here's what Channel islands say about it;

The CI Mid 2 + 1 was designed in collaboration between Southern Californian surfer Devon Howard and Channel Islands Surfboards, to enhance the CI mid-length’s high-performance characteristics. The set also proves versatile in a wide range of conditions with just a simple adjustment of the centre fin. 

Thanks to decades' worth of knowledge, we landed on this beautiful, tidy, well-balanced outline with full base and moderate rake that was hand foiled for medium flex—making for a very responsive ride with just the right amount of hold and lift that helps you accelerate through turns. Shaped from clear fiberglass panels, the CI Mid’s 6.5” centre box fin is available with either 4” side bites for medium to bigger sized surfers, or 3.6” for lighter weight riders around 160lbs and under.

• 3.6" side bites are designed for surfers 160lbs/72kgs and under.
• 4.0" side bites for are designed for surfers over 160lbs/72 kgs.