From the General Instruction of the Roman Missal and Appendix:

GIRM #21: For the sake of uniformity in movement and posture, the people should follow the directions given during the celebration by the deacon, the priest, or another minister. Unless other provision is made, at every Mass the people should stand … from the prayer over the gifts to the end of Mass, except at the places indicated later in this paragraph. [] They should kneel at the consecration unless prevented by the lack of space, the number of people present, or some other good reason.

Appendix:

At its meeting in November, 1969, the National Conference of Catholic Bishops voted that in general, the directives of the Roman Missal concerning the posture of the congregation at Mass should be left unchanged, but that no. 21 of the General Instruction should be adapted so that the people kneel beginning after the singing or recitation of the Sanctus until after the Amen of the Eucharistic Prayer, that is, before the Lord’s Prayer.

There are several points to be made in clarification:

1. The people are to follow the directions of the priest, deacon or other minister. The directions given by the said ministers should conform to those specified in the sentences following in #21 of the GIRM, including that one pertinent to the place reserved for kneeling at the consecration, as well as the adaptation made by the National Conference of Catholic Bishops in 1969 for the dioceses of the United States, which specifies that the people should kneel from the end of the Sanctus through the Amen concluding the Eucharistic Prayer, often called the Great Amen.

With regard to the exceptions noted pertaining to kneeling at the Consecration, lack of space, number of people or some other good reason, these points must be made:

• Removing the kneelers from a church does not constitute a valid exception from the directive to kneel at the specified times, especially at the Consecration.
• During periods of mission liturgy, planning and building of a permanent mission or parish church, or other periods of refurbishment or reconstruction, when kneelers are not present but anticipated, then the exception will be granted by me.
• In all but the most unforeseeable circumstances, any intention to relax the stipulation regarding kneeling during the Consecration should be communicated to me personally well beforehand. In other words, in this Archdiocese, the exception must be approved and granted by my authority.

In conclusion therefore, the people should kneel from the end of the Sanctus through the Great Amen.

2.With regard to the matter of some people wishing to kneel from the conclusion of the Agnus Dei through the distribution and reception of Holy Communion, the General Instruction views this time as a period of personal preparation of the priest, and bids the faithful to the same action. The General Instruction does not delimit this time-period, nor specify what particular posture is appropriate or not. What I have seen is that some people stand, some kneel and some sit, due to necessity in certain instances, With regard to the matter

It is my directive that within the Archdiocese of Atlanta, the people are free to kneel, sit or stand after the Agnus Dei, until all are invited to stand for the closing prayer and rite of dismissal.

3. For adults as well as children, the custom of the Roman Rite is to receive Holy Communion on the tongue. At the request of the National Conference of Catholic Bishops, and with the approval in 1977 of the Congregation for Divine Worship and the Sacraments, the optional practice of receiving Communion in the hand is now permitted, but with the provision that the option must remain the choice of the communicant. Under no circumstances can Communion be refused to worthy communicants wishing to receive on the tongue, nor should attempts be made to limit this choice, which would be contrary to the intent of the provision.

In keeping with the processional character assigned to the rite of Reception of Holy Communion by the 2^nd Vatican Council and the General Instruction, standing is the appropriate posture for receiving the Sacrament of Holy Communion. Also, the General Instruction speaks of a “proper reverence” being made by the communicant in approaching the priest or other minister of Communion, but does not define “proper reverence” any further. It must be assumed that the communicant is free to determine the style of the reverence, but always with the understanding that such reverences or gestures should not call attention to the self, nor cause disruption in the unity of the procession, and that the celebrant or other appropriate minister has the obligation to catechize those transgressing liturgical propriety, but not to the detriment of the rite, nor in such a way as to embarrass any communicant publicly. After making the appropriate verbal responses to the priest or minister, the communicant should return immediately to the appropriate place, no other action being called for by the rite.