Fins are movable structures that help a fish swim and keep its balance. A fish moves its fins by means of muscles. Except for a few finless species, all modern bony fish have rayed fins. Fish fins are classified according to their position on the body as well as according to their structure. Classified in this way, a fin is either median or paired.
Median fins are vertical fins on a fish's back, underside, or tail.
They include dorsal, anal, and caudal fins. The dorsal fin grows along the back and helps a fish keep upright. Almost all fish have at least one dorsal fin, and many have two or three. The anal fin grows on the underside near the tail. Like a dorsal fin, it helps a fish remain upright.