Article 1873-2 of the French Civil Code
The co-undividers, if they all consent, may agree to remain in undivided ownership. On pain of nullity, the agreement must be drawn up in writing including a description of the undivided property and an indication of the shares belonging to each undivided co-owner. If the undivided property includes claims, the formalities of article 1690 shall apply; if it includes immovable property, the formalities of land registration shall apply.