Resolution 1 of the 7th CGPM (1927)

Definition of the metre by the international Prototype

The unit of length is the metre, defined by the distance, at 0°, between the axes of the two central lines marked on the bar of platinum-iridium kept at the Bureau International des Poids et Mesures and declared Prototype of the metre by the 1st Conférence Générale des Poids et Mesures, this bar being subject to standard atmospheric pressure and supported on two cylinders of at least one centimetre diameter, symmetrically placed in the same horizontal plane at a distance of 571 mm from each other.

https://www.doi.org/10.59161/CGPM1927RES1E

NOTE

This definition was abrogated in 1960 by the 11th CGPM (Resolution 6).