Q: Is it permitted to place a cross carried in the entrance procession on the altar? I am trying to weigh up the following arguments.
Yes, because both GIRM 117 and 308 describe a portable cross that may be placed on the altar.
No, because GIRM 122 and 188 says the cross carried in procession may be placed near the altar, but does not mention on the altar. Ceremonial of Bishops, n, 129 and 194 says may be placed near the altar or be put away. For the Stational Mass in n. 129, Ceremonial of Bishops seems to only envisage the Book of the Gospels on the altar, not the candles or cross.
For Good Friday rubric 33 describes the cross being on the altar, but rubric 21 has the cross at the altar (although candles may be on the altar).
Further confusion: GIRM 277 envisages a situation where there is not a cross on the altar or near it: “The cross, if situated on the altar or near it, is incensed by the Priest before he incenses the altar; otherwise, he incenses it when he passes in front of it.”
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A: You’ve identified the significant passages, and, as you conclude, taken together, it is difficult to answer this question definitely. I’d say that the rubrics do not exclude placing a cross carried in procession upon the altar, but they more commonly envision that the cross carried in procession is of such a design that it would not suit placement upon an altar.