Written documents exist that describe the musical culture of Rus'. The most popular kind of instruments in medieval Russia were thought to have been string instruments, such as the gusli or gudok. Archeologists have uncovered examples of these instruments in the Novgorod region dating as early as 11th century. (Novgorod republic had deep traditions in music; its most popular folk hero and the chief character of several epics was Sadko, a gusli player). Other instruments in common use include flutes (svirel), and percussive instruments such as the treshchotka and the buben. The most popular form of music, however was singing. Bylinas (epic ballads) about folk heroes such as Sadko, Ilya Muromets, and others were often sung, sometimes to instrumental accompaniment. The texts of some of these epics have been recorded. In the period of Muscovy, a distinct line was formed between the sacred music of the Orthodox Church and that of secular music used for entertainment. The former draws its tradition from the Byzantine Empire, with key elements being used in Russian Orthodox bell ringing, as well as choral singing. Neumes were developed for musical notation, and as a result several examples of medieval sacred music have survived to this day, among them two stichera composed by Tsar Ivan IV in the 16th century. Secular music included the use of musical instruments such as fipple flutes and string instruments, and was usually played on holidays initially by skomorokhs — jesters and minstrels who entertained the nobility. During the reactionary period of the Great Russian Schism in the 17th century, skomorokhs along with their form of secular music were banned from plying their trade numerous times, but despite these restrictions, some of their traditions survived to the present day.