The treaty was sealed with a marriage alliance between the Angevin and Capetian dynasties. John's niece Blanche, daughter of his sister Leonora and Alfonso VIII of Castile, married Philip's eldest son, Louis VIII of France (to be eventually known as Louis the Lion).
The treaty was sealed with a union of royal families through marriage, specifically between the Angevin and Capetian lineages. John's sister Leonora's daughter Blanche married Philip's eldest son Louis, who would later be known as Louis the Lion.