The principle of tariff unity "One company - one collective agreement" is not reflected in the law. It is applied by the Federal Labor Court (BAG) and has been the subject of legal disputes for more than seventy years. It is upheld in consistent case law, but is more controversial in literature today than ever before. The author describes the principle of tariff unity in its twofold manifestation, first in the individual employment relationship of each employee in cases of tariff competition, and then in the entirety of the workforce of a single company in cases of tariff plurality. The issue is illustrated through the recent tariff dispute between Deutsche Bahn AG and the German Locomotive Drivers' Union.