This chapter talks about private law and public law as if everybody knew what was meant when these words are being used about law. This probably holds true for lawyers and even law students, but not for the general population. The private/public dichotomy is closely related to, but not identical with another distinction: between so-called dispositive and mandatory law. The distinction between private and public law was explicitly recognized by the Romans, but that does not mean of course that it did not exist beforehand. The chapter presents some examples from the earliest Russian law, the Russkaia Pravda (RP). The private/public dichotomy in law is not some sort of mysterious and inherent quality of law which discover by meticulous scientific investigation. It is a device invented by lawyers at a certain moment, when they considered it convenient. Keywords: mandatory law; private law; public law; Russian law; Russkaia Pravda (RP)