Prussia, over the course of the Nineteenth Century, attained mastery over all the other states of Germany outside Austrian control, and as each insignificant principality submitted to Prussian control, it assumed Prussian law. While some (or even many) of these laws have been since abrogated, a law which has not been abrogated may still legitimately be claimed as a legal source. The only error (to my mind) is that it should be cited as "a Prussian law from 1717, which acquired dispositive force of law in 1871 when Baden and Württemberg acceded to Prussian authority when Germany was created, or at the very latest in 1909 when Württemberg resolved its final issues regarding education with the Reich".