Enrichment of collections, conservation and development are naturally interwoven in the definition of a patrimonial development policy at the municipal library of Rennes. Heavy with tradition, the patrimony enjoys a stability and renown without having to enter into competition with the objectives of the reading public. Each activity benefits from means reinforced by a difference of budgetary analysis. The financial results, recorded in two tables, one devoted to acquisition expenses, the other to those of micro-reproduction and binding, translate into a political will towards a patrimonial ambition. This is inscribed in a consensual framework in a town that considers patrimony as fundamental to identity.