A method of aligning arm reference systems of a multiple-arm measuring machine having at least two measuring units, each having a movable arm; the method including the steps of aligning the reference systems of the measuring units when setting up the machine, and periodically updating alignment of the reference systems by detecting, by means of one measuring unit, a reference member carried by another measuring unit and moved successively by the other measuring unit into a number of positions at an intersection of the measuring volumes of the two measuring units.