Abstract: An apparatus for certifying or calibrating a coordinate measuring machine. A plurality of elongated linear gage blocks, each of said gage blocks which have been certified as conforming in accuracy to a specified accuracy grade by a qualified measuring laboratory are mounted in a positioning device for selectively rotating the gage blocks about a vertical axis within a measurement volume of a coordinate measurement machine and selectively rotating the linear gage blocks about a horizontal axis within the measurement volume. The positioning device has a base for supporting the apparatus in the measurement volume, an upward extending column mounted on the base for rotating the gage blocks about the vertical axis, a holding device mounted on an upper portion of the column for mounting said gage blocks in the positioning device and rotating the gage blocks about the horizontal axis.