Today, manufacturing facilities are designed for mass-producing identical goods. Although they often have a certain flexibility, they are not fully changeable. Changes are associated with a high cost. Changeability allows producers to react more quickly to changing demand situations and to efficiently produce small quantities. The Digital Twin is a key concept for implementing the required changeability. In the reference project BaSys 4.0, funded by the German Federal Ministry of Education and Research (BMBF), we have developed a manufacturing concept that enables changeable production. Our open source middleware Eclipse BaSyx provides a reference implementation of the BaSys 4.0 concepts.