The Rubrics in General
1. The rubrics are the rules (laws, directions, suggestions) which are contained in the liturgical books for the right ordering of liturgical functions. For the most part, if not entirely, the rubrics are positive ecclesiastical laws, and so (a) they bind under pain of mortal or venial sin, according to the gravity of the matter with which they are concerned; (b) apart from such considerations as the giving of scandal, contempt for the law, and the like, a sufficient and proportionately grave cause excuses from the observance of an (accidental) rubric.
2. The rubrics of the Missal comprise (a) the General Rubrics (new Code of July 26, 1960), (b) the rite of the actual celebration of Mass, (c) the defects that may occur in its celebration, (d) the special rubrics scattered throughout the Missal intercalated in the text of the actual formularies of the Mass, and especially in the Ordo and Canon (where they form a synopsis of the rubrics of the Ritus). It is these special rubrics that are, ordinarily, printed in red (ruber), so that they may easily be distinguished from the prayers and reading.
3. The rubrics of the liturgical books are classified as:
a) Substantial and accidental, the former being those that prescribe the matter or form of a sacrament (e.g., the imposition of hands in Confirmation, or at the ordination of a priest), or regulate the validity of a liturgical act (e.g., the anointing at the consecration of an altar or of a chalice); the latter are rubrics which do not concern the validity of a liturgical act, but regulate the rite in which it is enshrined. Substantial rubrics are based on divine law and, for the sacraments and Mass, are immutable; the accidental rubrics are purely ecclesiastical laws, and may be changed or dispensed from, but only by the Holy See.
b) Preceptive, directed, facultative. Preceptive rubrics are those which bind in conscience and are real laws; directive rubrics are those that do not per se bind in conscience, but set forth–by way of counsel or direction–the approved way of carrying out a liturgical act; facultative rubrics are those which permit of a choice between two courses, or between doing or omitting something, or which allow liberty to follow existing usage.
4. Obviously, substantial rubrics are binding in conscience, sub gravi, while facultative rubrics do not bind at all in conscience. The question that has been hotly disputed by theologians and rubricians, since the end of the sixteenth century, is whether accidental rubrics are all preceptive or not.
The Obligation of Accidental Rubrics
5. While some rubricians and moral theologians hold that not all accidental rubrics are preceptive, that some are directly only, the better opinion seems to be that all such rubrics are preceptive, except, obviously, when they are facultative. All rubrics are ecclesiastical laws, and laws do not direct, they prescribe, forbid, or permit. And so accidental rubrics bind in conscience, but like all laws the extent of the binding force of any particular rubric and the obligation of obedience to it depend on the nature of the law which the rubric embodies, and such other considerations as govern the observance of any ecclesiastical law. And as laws, rubrics are subject to the general canon law on custom as set forth in Canons 25 to 30 of the Code of Canon Law.
6. Whether each single rubric (except the facultative ones) binds in conscience or not is, then, an open question. What all theologians and rubricians are one in believing and teaching is that Sancta sancte tractanda sunt–that the object of every rubric is to aid the priest in performing as perfectly as possible those sublime acts of worship whose end is to honour God worthily and edify men. It is the spirit of the rubrics, in the long run, rather than the letter of them, that is important. With reverence and love towards God, out of obedience and loyalty to his Church, should they, each and all, be fulfilled.
[From The Celebration of Mass, A Study of the Rubrics of the Roman Missal, by Rev. J.B. O’Connell]