This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Open a printed Roman Missal and the first substantial text a reader meets is not the Mass itself but a document about the Mass: the General Instruction of the Roman Missal, a governing preface that explains how the book is organized, what each of its parts is for, and the principles behind the rubrics that follow. It is easy for that positioning to blur the two into one another, especially since the Instruction is often the passage cited when a reader wants to know how the Missal works rather than what it actually prays, but they are not the same text. The Missal is the prayer book itself, the Order of Mass and the full cycle of proper texts assigned to each day of the liturgical year; the Instruction is the Church's own commentary on that book, bound together with it for the reader's convenience.
The distinction matters historically as much as structurally. The Missal's own lineage runs through named promulgations, Pius V's Tridentine edition in 1570 fixing the Roman Rite's text after the Council of Trent, Paul VI's 1970 revision after the Second Vatican Council, and the third typical edition issued in 2000 and 2002, each one a new authoritative Latin text from which vernacular translations are then prepared and confirmed by Rome. The General Instruction has its own parallel history of revision alongside the Missal it introduces, updated to reflect each Missal edition's own changes.
Distinguishing them is less a technicality than a reminder that a liturgical book, like scripture in any tradition, tends to travel with its own commentary attached closely enough that later readers can lose track of which words are the primary text and which are the guide written to explain it.