Butterfly Bettas have a solid colored body, and their fins are divided into two separate colors, which are ideally symmetrical. The fins display a banded pattern with emphasis placed on the contrast of the band rather than the coloring of the body and fins. The bands should be crisp - not a blending of opposing fin color but one with an equal division between one color and another on the fins. The variegated fin mutation is dominant but the effects are highly variable from fish to fish.
The bands should be of more or less equal width (ideally 1/2 the area of the total fin) and symmetry. Lack of definition between the bands is considered a fault. There are two forms the BF may take: The two-band fin pattern, and the multiple-band fin pattern. Bands should always be relatively equal in width and breadth regardless of their number, and have clear separation of color.
Here are two butterfly patterns -