 Les Innocents had begun as a cemetery with individual sepulchres, but by then had become a site for mass graves.People were buried together in the same pit (a pit could hold about 1,500 dead at a time); only when it was full would another be opened.In the 14th and 15th centuries, citizens constructed arched structures called "charniers" or charnel houses along the cemetery walls to relieve the overcrowding of the mass graves; bones from the graves were excavated and then deposited here.