Harlequin is a white marking that presents as a series of vaguely diamond-shaped markings connected at the corners.

Harlequin tends to "radiate out" from a single place, with rings of connected diamonds all pointing towards one point, usually the poll or the croup.
Diamond markings vary in size but must be no smaller than the horse's eye and no larger than the horse's head.
Diamonds may be disconnected from the rest of the Harlequin pattern as long as they are oriented in the same direction as the rest of the marking.
Most diamonds should be connected, however "breaks" or gaps in rings of diamonds are acceptable.
Harlequin markings will never have jagged edges, holes, or spots.

Genotype: Hq_
Locus:  Not shared with any known genes

• Coat: Harlequin is a white marking.
• Mane and Tail: White where touching marking.
• Skin: Pink under marking.
• Eyes: Can cause blue eyes.
• Hooves: Not affected.

Range:

• Minimum: Two white diamond-shaped markings connected at the corners. The diamonds must be somewhere on the barrel or neck, and must be at least the size of the horse's eye.
• Maximum: A pattern of diamonds that covers the entire neck, head, barrel, and hindquarters. Up to two legs (either both forelegs or both hind legs) may have diamond markings.

Interactions:

• Chimera & Somatic: Chimera and Somatic markings have an unusual tendency to "fill" the spaces between Harlequin diamonds. Additionally, horses with both Harlequin and the Chimera anomaly can sometimes display two different colors of Chimera markings.