The tail fin of a fish. This is the only fin that helps significantly with forward propulsion; the other fins are used for stabilizing and subtle maneuvering.

The shape of the caudal fin predicts how fast a fish tends to swim. Fishes with V-shaped or U-shaped fins tend to be fast swimmers (e.g., tuna); fishes with more rounded fins tend to be slow (e.g., gar).

Ichthyologists classify caudal fins according to shape. The primary types are:

Note: some authors classify every eel-like tail as protocercal. They're ignoring the internal structure and focusing only on the general form. Current evolutionary theory hypothesizes that protocercal fishes appeared first, then heterocercal, abbreviated heterocercal, homocercal and finally isocercal. (Diphycercal and hypocercal are uncertain offshoots.) Despite their shape, eels have very little in common with lampreys.