The prescribed liturgy for celebrating the Sacrifice of the Eucharist in the Roman Rite. Essentially this ritual consists of two parts: the Liturgy of the Word and the Liturgy of the Eucharist. "In the Mass, both the table of God's word and the table of Christ's body are prepared, so that from them the faithful may be instructed and nourished" (Paul VI, General Instruction on the Roman Missal, 8). There are also some introductory and concluding rites.
Accordingly the Mass ritual is divided into four parts: Introduction, Liturgy of the Word, Liturgy of the Eucharist, Conclusion. While in general the ritual is carefully prescribed, there are also some options left to the discretion of the priest as presiding celebrant.