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Commentating on the 1917 Code, Dom Augustine writes in his A Commentary on the New Code of Canon Law can. 856:
No one who is conscious of a mortal sin, no matter how sorry or contrite he may feel, is allowed to receive Holy Communion without having previously gone to confession. In case of urgent necessity, when no suitable confessor is at hand, such a one must make an act of perfect contrition before approaching the Sacred Table.
This canon we leave to moralists to explain, because it pertains to the court of consequence. We will only add that this law, no matter whether it be regarded as divine [cf. 1 Cor. 11:27,29] or ecclesiastical, is a grave one, as is apparent from the Council of Trent [cf. sess. 6, ch. 15]. Copia confessarii must be understood of any confessor with the necessary faculties who is not an accomplice of the penitent. Theologians say that the repugnance to, or impossibility of going to confession must be such as is not directly connected with the act itself. Urgent necessity of receiving Holy Communion exists when one has to fulfill the paschal obligation, and before contracting marriage.
The New Commentary on the Code of Canon Law p. 1170 on the 1983 Code says:
It is not for canon law to determine what sins, including transgressions of the ecclesiastical law itself, are grave and what sins are non-grave or venial.89
89. β¦canonical avoidance of distinctions between "grave" and "mortal" sinsβ¦
Thus it seems "grave" and "mortal" sins are synonymous terms in Canon Law.