Abstract: An instrument permitting linear dimensions to be measured to four or more significant decimal digits includes a stack of identical gage blocks movable in a receptacle in the direction of stack elongation and a strong spring longitudinally compressing the stack. A carrier pin on the housing of a micrometer may be inserted between longitudinally consecutive blocks of the stack to establish the first digit. Camming cooperation of the carrier pin with the gage blocks draws engagement faces on the micrometer housing against guide faces of the receptacle. The contact member of the micrometer is mounted on the micrometer slide which may be shifted longitudinally of the stack by engaged threads on the slide and on a drum rotatable on the housing about a longitudinal axis. The second digit is read from cooperating indicia on the housing and slide, and the third and fourth digits from indicia on the drum and on the housing.