Culprit Surf Honeycomb RTM FCS M3 Style Thruster Surfboard Fins Different fin sizes (C1, C2 C3 and C4) for different weight ranges to optimized performance FCS Compatible Laminated fiberglass for a super stiff construction Light weight honeycomb foam core=reduced weight, more speed Excellent balance of base length, height and sweep gives these fins good drive, grip and optimum maneuverability.