Following the rare burnings by fire (by household) of the Ancien Régime, the French administration of the 19th century sets up nominative censuses of the population every 5 years from 1836, the years ending in 1 and the years ending in 6 (except the years of war).

By department and cities, then by household, these individual nominative counts make it possible to follow the demographic evolution of the country throughout the century. They make it possible to establish a "" photograph "" of the focus - and not only of the nuclear family - at the exact moment.

They help to discover, for example, all siblings or cousins, grandmothers living at the home of their children or grandchildren. For the department of Côtes du Nord (now Côtes d'Armor), there are, for the 7 censuses carried out in 1872, 1876, 1881, 1886, 1891, 1901 and 1906, an annual average of 130 000 households.