The document discusses the troparia and kontakia, which are variable hymns sung in the Byzantine liturgy. It explains that on ordinary Sundays, one of eight series of resurrectional troparia and kontakia is sung, following the proper musical mode or tone for that week. It also describes how troparia and kontakia can be specific to saints, days of the week, or special occasions. Precise rules govern how different sets are combined based on liturgical precedence. The origins of Christian hymnody drew from Hebrew psalms and incorporated new types of hymns over time, though private compositions were later discouraged due to heresy risks.