This is a quad fin back fin (rear fin). For setting changes from tri-fin to quad-fin, etc. M size, brand new, unused. Thank you. #FCS2 #FCS #Performer #Surfing #Surfboard #Fin #QuadFin #QuadFin #Shortboard Accessories...Fin Type...Fin Fin System/Fin Box...FCS2 Number of Fins...Quad Fin