Dimensional metrology has always played an important role in society. Indeed, with the increase of international trade seen in the 19th century, the need for international agreement on length and mass measurements formed the starting point for the global homogenization of units as embodied in the Metre Convention.
Over the years, the definition of the metre has served as a model. The International Prototype of the Metre was one of two International Prototypes deposited at the BIPM headquarters in 1889. In 1960 the metre became the first SI unit to be defined based on a quantum phenomenon, and in 1983 it became the first SI unit to be defined by fixing the numerical value of a fundamental constant: the speed of light. Today, adoption of similar definitions for other SI base units seems to be the way forward in the planned future global revision of the SI.
Today, the realization of the metre is essentially based on laser frequency standards, which are increasingly accompanied by techniques to directly generate optical frequencies from the second in terms of optical frequency comb systems.
From the early seventies through till 2006 the BIPM piloted a long line of laser frequency comparisons, providing traceability and validation of national length standards. Although today there is no active technical programme in length measurements at the BIPM headquarters, international agreement in length metrology remains essential and the BIPM continues to coordinate international agreement in this area, cooperating with the CCL to provide the basis for homogeneous realization of the metre world-wide.