The first classification system was developed by O'Connell in 1976, where<unk>  were classified as total and partial. In the former, twins shared an "extensive surface area with widely connected cranial cavities", while in the latter, only a "limited, superficial surface area" was connected. In 2006, Stone and Goodrich came up with a more nuanced classification system, which is the "most commonly used" system. Partial CPT was defined as "lacking substantial shared dural venous sinuses", whereas total CPT "share a large portion of their dural venous sinuses and present with pronounced brain compression, which leads to distortion within the cranium". Total Total<unk>  share a large portion of dural venous sinuses and present with pronounced brain compression, leading to distortion within the cranium. Stone and Goodrich also defined two main subtypes, based on whether the long-axis angle between the twins was angular or vertical. The angular subtype has a higher rate of comorbidities than the vertical subtype. Vertical<unk>  calvaria is continuous and is further subdivided on the basis of intertwin axial facial rotation: *Type 1: both children face in the same general directional axis so that the angle between twins is less than 40 degrees. These twins show relatively symmetric superior bi-parietal or vertex compressional flattening. *Type 2: both children face opposite directions so that the deformity shows an axial rotation between 140 and 180 degrees. *Type 3: in this variety axial rotation is intermediate between the first two types with a rotation of being between 40 and 140. Another classification system divides total<unk>  into four categories: * Frontal: twins are facing each other with the axis of the bodies forming an acute angle * Temporoparietal: joined immediately above the external auditory meatus * Occipital: twins are connected in the occipital lobe causing the twins to face away from each other * Parietal: twins fuse at the vertex with the axis of the twins forming an obtuse angle; this leads to a situation where the twins share the most veins, lobes, and neural circuitry and thus is often described as one brain shared by two individuals Occipital lobe Having this kind of juncture means that there would be one common continuous cranium housing four cerebral hemispheres. An incomplete dural septum typically separates the flattened cerebral hemispheres. In total vertical<unk> , the major cerebral arterial supply is usually confined to each respective twin and in some cases conjoined brain tissue may contain a larger artery. Within this category there are three smaller subdivisions that basically outline the different rotational symmetry of the junction. Partial Stone and Goodrich define partial<unk>  as lacking substantial shared dural venous sinuses, with limited surface area involvement, with either intact crania or cranial defects. It is less common than total<unk> . In partial<unk> , the unions are usually frontal and less commonly occipital and vertical. Angular frontal junctions occur when the two twins are joined at any part of the forehead. Occipital twins are joined at the occipital lobe in the back of the head and vertical are joined on the top of the head and usually face opposite directions. The junctional diameter is often smaller in partial forms and occasionally an incomplete layer of bone may be present between the twins. Each child maintains independent calvarial convexities except at the common area of skull junction. The dura of both children may be intact or deficient and cortical gyri may interdigitate. Additionally, shared dural venous sinuses is usually absent, or, if it is present, negligible. These twins usually undergo successful separation and both twins may live to lead normal lives.
Craniopagus twins
Monoamniotic twins
Conjoined twins
Monochorionic twins
Vanishing twin
Vestigial twin
Anencephaly
Locked twins