In the 15th – 16th centuries the idea of angel-voice singing associated with monothematic unison singing was reconsidered. It happened along with the shift in Russian iconography to the concept of Trinity actively developed since the 15th century. Just like Andrey Rublyov's Trinity became the highest expression of the theological doctrine, so the idea of trinity was expressed in the Russian church music with the special form of polyphony named the 3-line hymnody. For this type of singing the voices were written serially in lines one over another making a multi-colored score. The main voice was the “way”, i.e. the middle voice leading the Znamenny (plain) chant melody. It was framed with the higher “top” and lower “bottom” voices. For long time Russia kept the custom of entrusting the most important chants to three young men. The prototype of 3-lyne hymnody was probably the biblical story from the Book of Daniel about three adolescents, who did not bow to the gold idol; they were punished by the Babylonian tsar Nebuchadnezzar, who plunged them into a fiery furnace, where the three sang thanksgiving to God and were rescued by angels that came down from the sky.