The obligation of specific written approval of the arbitration clause under art. 1341 co. 2 cod. civ. presupposes the concurrent existence of two requirements: the unilateral drafting of the contractual terms by one of the contracting parties and the intended use of the contractual scheme to govern an indefinite series of relationships. The formal standardisation of the contractual scheme does not amount to drafting for an indefinite series of relationships where the contracts are functionally linked to the completion of a specific transaction and the counterparties are identified by reference to the specific expertise required.