The present Roman Missal, now almost universally used in the Catholic Church wherever the Latin Rite prevails, consists essentially of two parts of very unequal length. The smaller of these divisions containing that portion of the liturgy which is said in every Mass, the “Ordo Missae” with the prefaces and the Canon, is placed, probably with a view to the more convenient opening of the book, near the centre of the volume immediately before the proper Mass for Easter Sunday. The remainder of the book is devoted to those portions of the liturgy which vary from day to day according to feast and season. Each Mass consists usually of Introit, Collect, Epistle, Gradual and Alleluia or Tract, Gospel, Offertory, Secret, Communion, and Post-Communion, the passages or prayers corresponding to each of these titles being commonly printed in full. (https://www.newadvent.org/cathen/10354c.htm)
