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The double tail variety has two fin lobes rather than one. Also, the dorsal (top) fin has almost twice as many rays as on a single tail (ST) Betta, which results in a much bigger fin. Ideally, both lobes of the tail must be symmetrical. When flaring, the fins almost make a full circle. The DT trait is recessive to ST, and therefore rather elusive. ST bettas who carry the DT gene have dorsal fins that are larger than those ST that don’t carry this gene. If you find the DT trait not appealing, like some breeders do, make sure to ask about the DT gene before purchasing a fish for breeding purposes.