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A typical adult human skeleton consists of 206 bones, not counting many small and often variable sesamoid bones and ossicles. Individuals may have more or fewer bones than this owing to anatomical variations. The most common variations include additional (i.e. supernumerary) cervical ribs or lumbar vertebrae. Sesamoid bone counts also may vary among individuals. The figure of 206 bones is commonly repeated, but must be noted to have some peculiarities in its method of counting. As noted below, the craniofacial bones are counted separately despite the synostoses, which occur naturally in the skull. Some reliable sesamoid bones (e.g., pisiform) are counted, while others (e.g., hallux sesamoids) are not. The count of bones also changes with age, as multiple ossific nuclei joined by synchondroses fuse into fewer mature bones, a process which typically reaches completion in the third decade of life.