A thumb with three phalanges in a single, proximo-distal axis. Thus, this term applies if the thumb has an accessory phalanx, leading to a digit like appearance of the thumb. The phenotype is HP_0001199 (Triphalangeal thumb). Also known as: Accessory phalanx of the thumb, Digitalized thumb, Finger-like thumb, Triphalangeal thumbs, Triphalangy of thumb.