Most "strong" models are optional, i.e. the parents can choose the bilingual school alternative, but there are also cases in which bilingual teaching is firmly integrated in the curriculum for linguistic and educational policy reasons, and cannot be selected or rejected (the Romansh-speaking part of the Canton of Grisons, Luxembourg, Aosta, Catalonia). ###>Certains pays et régions appliquent des transitions progressives entre les langues (partie francophone des Grisons, Luxembourg, Singapour).