A partial duplication, depending on severity leading to a broad or bifid appearance, affecting one or more of the phalanges of the thumb. As opposed to a complete duplication there is still a variable degree of fusion between the duplicated bones. The phenotype is Human Phenotype Ontology id HP_0009944 (Partial duplication of thumb phalanx). Also known as: Bifid thumb, Notching of thumb phalanges, Partial duplication of the phalanges of the thumb, Partial duplication of the thumb bones.