Webbing or fusion of the fingers or toes, involving soft parts only or including bone structure. Bony fusions are referred to as "bony" syndactyly if the fusion occurs in a radio-ulnar axis. Fusions of bones of the fingers or toes in a proximo-distal axis are referred to as "symphalangism".

Supernumerary digits located at the ulnar side of the hand (that is, on the side with the fifth finger).

A soft tissue continuity in the A/P axis between two fingers that extends distally to at least the level of the proximal interphalangeal joints, or a soft tissue continuity in the A/P axis between two fingers that lies significantly distal to the flexion crease that overlies the metacarpophalangeal joint of the adjacent fingers.

Fusion involving two or more metacarpal bones (A synostosis of the first metacarpal and the proximal phalanx of the thumb can also be observed, note that the first metacarpal bone corresponds to a proximal phalanx).