A subdivision of the national territory in homogeneous zones deriving from the aggregation of neighbouring municipalities, normally built on the base of altimetric threshold values. Mountain, hill and plain altimetric zones are distinguished.

A territory characterised by widespread relief masses, normally less than 600 metres high in northern Italy and 700 metres high in central and southern Italy and in the Islands. Possible small enclosed areas having different characteristics are considered as part of the hill zone.

A territory characterised by widespread relief masses, normally not less than 600 metres high in Northern Italy and 700 metres high in Central and Southern Italy and in the Islands. These threshold levels can be changed according to the lower limits of the Alpinetum, Picetum and Fagetum phytogeographic zones, and according to the upper limits of mass vine cultivation in northern Italy and olive cultivation in central and southern Italy and in the Islands. Areas enclosed between the relief masses, having plain, upland and similar soil shaping are considered as part of the mountain zone.

A low and plain territory characterised by the absence of relief masses. Parts of territory which, in their zones farthest from the sea, raise to an height which, normally, is not above 300 metres are also included in the plain zone, provided they have an overall inclination continously negligible with respect to the rest of the plain zone. Valley floors open to the plain beyond the river fan peak are excluded from the plain even if they are flattened. Small coastal plain strips are also excluded. Possible negligible mountain or hill reliefs included in the plain surface are considered as part of the plain zone.

For each municipality, it is made up by the persons usually living in the municipality, even if at the time of census they are absent because they are temporarily present in another italian or foreign municipality.