Twin Surf Fins
A twin setup is usually found on retro boards. This fin setup makes your board very loose and manoeuvrable. Twin fins have two large fins at the side of the bottom of a board, which are foiled. This gives your board sidewards pressure and more grip on the rails. This is why boards with a short rail line, or very round boards with little effective rail, usually have a twin fin setup. This setup gives the board more grip. Besides this, the absence of a fin in the middle, enables the water to flow through easily. This allows for more speed, decreases the amount of control though. For example, check out the twin fins of Future fins, FCS and Feather!
The inspiration for the twin fin setup came from the Paipo fins (classical Hawaiian bodyboards). Bob Simmons used these already in the 50s, but Mark Richards made them famous by winning two world titles with these fins in the 70s. Nowadays the twin fin setup is a popular phenomenon.
Check the Ultimate Surfboard Fin Guide for more information!