To be inscribed on the UNESCO World Heritage List, UNESCO World Heritage properties must be of Outstanding Universal Value and meet at least one of ten selection criteria. La Chaux-de-Fonds and Le Locle, Watchmaking Town Planning, meet criterion IV for its inscription.
Criterion (iv): La Chaux-de-Fonds and Le Locle constitute a unique urban and architectural ensemble, wholly dedicated to watchmaking from the 18th century until the present day. Watchmaking space and living space co-exist in an extremely close relationship. The rational, pragmatic, and open planning of the urban space has encouraged the sustainable development of this mono-industry, as a "manufacturing-town".
Integrity and authenticity: The integrity of the watchmaking vocation of the two towns of La Chaux-de-Fonds and Le Locle is total and has remained so for more than two centuries; furthermore, this vocation is still active. It is given concrete expression in the permanence of the ordered and cumulative street plans of the first half of the 19th century and the continuity of the basic architectonic motifs of the built structure, based on a comprehensive typology from the end of the 18th century until today. The typological and environmental study of post-1930 buildings shows some important disruptions (high buildings) but above all, functional and architectural continuity (factories of the 1960s, workers’ housing estates) with the earlier built structure. The numerical indexes based on precise data in the evaluation of the integrity and authenticity of an urban ensemble are useful.
You can find further information at whc.unesco.org.