key·stone
noun/ˈkiːstəʊn/
/ˈkiːstəʊn/
- (architecture) the central stone at the top of an arch that keeps all the other stones in position
拱顶石 - [usually singular] the most important part of a plan or argument that the other parts depend on
（计划、论据的）主旨，基础 - Changes to the welfare system are the keystone of the government’s reforms.
福利制度的变革是政府改革的重点。
- Changes to the welfare system are the keystone of the government’s reforms.