The term “in perpetuum”, or “perpetuo”, meaning “forever”, “continuously”, “constantly”, passes into the practice of medieval Italian cities from Roman law, which had an undeniable influence on the law of the Middle Ages. It seems, however, that in the notarial acts of Northern Italy of the 12th — 14th centuries. This term, most often found in contracts of sale, is used in a special sense and has a special meaning. When analyzing purchase and sale agreements, it is important to determine which rights to the acquired property are granted to the buyer by such contracts, called “venditio — sale” by their notaries. The study of the acts shows that the presence of “in perpetuum” in them indicates that the property has been transferred on the rights of ownership. On the contrary, contracts that have the self-designation “venditio”, but do not include “in perpetuum” in their composition, grant the buyer lesser, limited rights. Thus, the presence or absence of the category “in perpetuum” in the document is of practical importance for the study of medieval act material, in particular, for its attribution.