According to American Dahlia Society standards
Formal Decorative: Ray florets are generally flat, broad, and smooth in a regular arrangement that gradually recurve toward the stem. Each ray floret row should convey uniform and regular size arrangement with the tips preferably round or extending to a slight point. The ideal depth is three-quarters the diameter of the bloom and should not be greater than its diameter.
Informal Decorative: Ray florets are twisted, curled, or wavy, and of uniform size in irregular arrangement. They may be partially involute or revolute, but no portion should be fully revolute or involute except at the tip of the ray floret. The ideal depth is three-quarters the diameter of the bloom and should not be greater than its diameter.
Semi-Cactus: Ray florets are broad at the base, straight or incurved, and reflex toward the stemin a regular and uniform arrangement. Ray florets will be revolute for approximately half their length and fully revolute (touching or overlapping) for at least one-fourth their length. The ideal depth is three-quarters the diameter of the bloom and should not be greater than its diameter.
Straight Cactus: Ray florets are narrow at the base, straight, uniform in length and reflex toward the stem, radiating uniformly in all directions from the center. They will be revolute for the majority of their length and fully revolute (touching or overlapping) for approximately half their length. A depth of about three-quarters the diameter is ideal for this form.
Incurved Cactus: Ray florets are pointed, uniform in length; they uniformly curve toward the face of the bloom, are revolute for the majority of their length and are fully revolute for at least half their length. Because ray florets curve toward the face ofthe bloom, they usually lack the depth of the preceding dahlia forms. A depth of one-half the diameter or slightly more is ideal for this form.
Laciniated: Ray florets are split at the tip with the number and depth of the splits varying according to the size of the ray floret. The portion of the ray floret with the splits should uniformly twist or curl whether involute or revolute. The less mature ray florets should possess splits but do not have to twist or curl. In the ideal laciniated dahlia, the majority of ray florets will be uniformly arranged with a split, twist, or curl that conveys an overall fringed or frilled effect. Ray florets, slightly involute or revolute, that split regardless of length and remain generally flat are a major fault. Ray florets with a shallow notch or cleft and possessing little twisting to the extreme portion should not be classified as laciniated. The ideal depth is three-quarters the diameter of the bloom and should never be greater than its diameter.
Ball: Blooms are ball shaped with uniform florets, involute for most of their length and fully involute for more than half the length; the ray florets reflex toward the stem. Ray floret tips in cross-section should be circular and exhibit little distortion, completely filling the floral head and are either round or blunt without notch or point.
Miniature Ball: Except for size, miniature ball and ball dahlias possess the same form features.
Pompon: Except for size, they are similar to the ball description.
Waterlily: The bloom should be fully double and symmetrical; the sideview should be flat to saucer-shaped in arrangement, and the layer of florets should be open faced to give a delicate appearance. The center should be closed and dome-shaped, breaking gradually to four to seven rows of fully developed outer florets. Outer florets should be broad and slightly cupped with rounded tips. A bloom position at 45 degrees is preferred, but a top facing bloom is acceptable. In petite waterlily cultivars (from 1 to 3 inches in diameter), the depth should contain fewer rows of ray florets in order to represent the waterlily form accurately. The depth should never be more than one-half the diameter of the bloom.
Peony: At least two, but not more than five rows of ray florets surround the disc. Ray florets are broad, generally flat to slightly cupped and evenly spaced in a flat plane. Ray florets are equal or nearly equal in length from the bloom face to the back, not recurving or reflexing to the stem. Ray florets adjacent to the disc may be twisted or curled, but if present should be symmetrically arranged around the disc. Ideal depth is less than one-third the diameter. Disc florets may show zero to two rows of pollen.
Anemone: One or more rows of ray florets surround a center of elongated tubular disc florets. The outer ray florets should be equal in shape, size and formation, generally flat with rounded tips and regularly arranged around the disc florets. Greater visibility of the ray florets from the face is preferred. The tubular disc florets should be fully developed and present a domed, pincushion appearance.
Collarette: A bloom will have a single row of flat or slightly cupped ray florets arranged in a flat plane; the ray florets will uniformly overlap, preferably in the same direction with no gaps. The petaloids surrounding the disc should be approximately one-half to two-thirds the length of the ray florets. Eight ray florets are most desirable. A round floret tip is preferred. The disc flower should contain evidence of pollen with up to three rows desirable.
Single: The bloom has a single row of flat or slightly cupped ray florets arranged in a flat plane. They uniformly overlap, preferably in the same direction with no gaps. Eight ray florets are most desirable. Disc flowers with up to three rows of pollen are desirable. A round floret tip is preferred.
Mignon Single: Mignon single dahlias possess the same formation as single dahlias except they have round ray floret tips. Disc flowers should have no more than two rows of pollen.
Orchid: A single row of uniform, evenly spaced ray florets arranged in a flat plane surround the disc flowers. The ray florets should be straight and involute for two-thirds or more of their length and fully involute for at least one-third their length. Eight ray florets are most desirable. Disc flowers with up to three rows of pollen are desirable.
Novelty Fully Double: Dahlias with characteristics distinct and different from the present classifications are classified as novelty dahlias. The center should be closed.
Bicolor: Two distinct clear and sharply separated colors on the face of the ray florets with the tip or central length contrasting.
Cultivar: Cultivated variety.
Dark Blend: Blends of low brilliance including dark reds and purples.
Dinnerplate: Traditionally, a dahlia variety that measures 10 inches or larger across.
Fimbriated: Having a fringe or border of hairlike or fingerlike projections.
Flame Blend: Blends of the light and high brilliance shades of red or orange with yellow.
Floret: One of the small flowers forming the head of a composite plant.
Foliage: The aggregate of leaves on one or more plants.
Involute: Curled or curved inward.
Light Blend: Blends of the lighter tints and tones of pink, yellow, lavender, and other pastels; also two-toned varieties of pastel tints and tones in which the central rays are of a different color than the marginal rays.
Petaloid: Florets having the form and appearance of smaller petals.
Revolute: Rolled backward or downward.
Variegated: Having patches, stripes or marks of different colors.