7th century to create Arab music as a synthesis of Byzantine and Persian music, and these exchanges were continued through the Ottoman Empire until Istanbul today. The term Byzantine music is sometimes associated with the medieval sacred chant of Christian Churches following the Constantinopolitan Rite. There is also an identification of "Byzantine music" with "Eastern Christian liturgical chant", which is due to certain monastic reforms, such as the Octoechos reform of the Quinisext Council (692) and the later reforms of the Stoudios Monastery under its abbots Sabas and Theodore. The triodion created during the reform of Theodore was also soon translated into Slavonic, which required also the adaption of melodic models to the prosody of the language. Later, after the Patriarchate and Court had returned to Constantinople in 1261, the former cathedral rite was not continued, but replaced by a mixed rite, which used the Byzantine Round notation to integrate the former notations of the former chant books (Papadike). This notation had developed within the book sticherarion created by the Stoudios Monastery, but it was used for the books of the cathedral rites written in a period after the fourth crusade, when the cathedral rite was already abandoned at Constantinople. It is being discussed that in the Narthex of the Hagia Sophia an organ was placed for use in secular processions of the Emperor's entourage. The earliest sources and the tonal system of Byzantine music According to the chant manual "Hagiopolites" of 16 church tones (echoi), the author of this treatise introduces a tonal system of 10 echoi. Nevertheless, both schools have in common a set of 4 octaves (, and ), each of them had a (authentic mode) with the finalis on the degree V of the mode, and a (plagal mode) with the final note on the degree I. According to Latin theory, the resulting eight tones (octoechos) had been identified with the seven modes (octave species) and tropes (tropoi which meant the transposition of these modes). The names of the tropes like "Dorian" etc. had been also used in Greek chant manuals, but the names Lydian and Phrygian for the octaves of and had been sometimes exchanged. The Ancient Greek harmonikai was a Hellenist reception of the Pythagorean education programme defined as mathemata ("exercises"). Harmonikai was one of them. Today, chanters of the Christian Orthodox churches identify with the heritage of Byzantine music whose earliest composers are