A CMM of the type Gantry uses three moving elements that traverse via pneumo-static bearings along as many guideways perpendicular to each other.
The touch probe is attached to the quill element, which translates vertically relative to the head element. The latter translates horizontally with respect to the crosshead element, which, in turn, translates horizontally with respect to the machine plane. The crossmember is motorized by a pinion-rack system along two guides, called stringers, located high above the machine plane. A colonnade system supports these rails and releases their longitudinal expansion through a system of leaf springs.