Complex sentence: Because of the complexity of the system, in the 16th century appeared so called "kinovarnye znaki", small letters printed in red before each sign, to indicate the first note of the sign it stood before.
Split into simpler sentences: Because of the complexity of the system, a simplified system was developed in the 16th century called "kinovarnye znaki", where small letters were printed in red before each sign. These indicated the first note of the sign it stood before.