in the us federal prison system, solitary confinement is known as the special housing unit (shu), pronounced like "shoe" (). california's prison system also uses the abbreviation shu, but it stands for security housing units. in other states, it is known as the special management unit (smu). current estimates of the number of inmates held in solitary confinement are difficult to determine, though generally the minimum held at any given time has been determined to be 20,000, with estimates as high as 80,000.