A carriage assembly for a metrology apparatus comprises an aperture 314 and a channel 325, the aperture being configured to allow a datum rail 322 having a longitudinal axis to slide therethrough and the channel being configured to allow a guide rail 324, parallel to the datum rail, to slide therethrough. At least one of the aperture and the channel comprise a biasing arrangement 375 integrally formed with the carriage assembly for inhibiting movement, such as rotational movement, of the carnage assembly other than along the longitudinal axis of the datum rail or guide rail. The carriage may be injection moulded from plastics and is preferably formed as a single, unitary piece. The biasing arrangement may comprise a spring arm.
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