As far as I am aware, the posture of the laity during Mass has never been been determined by the rubrics. Rather, it was always a matter of custom.
This is correct. There are no official rubrics for the laity.
There are rubrics for the postures of the laity at low Mass, as described below. The “Lasance” Missal contains a card which describes the postures on page 72 (on my edition) entitled “When to kneel, stand, and sit at Low, High, Requiem or Solemn Mass according to the custom
in America. This is usually as far as people go when quoting Lasance, however there are more details on the footnotes of the card, and additionally much more
"international" details on pages 69 and 70. Notice that in all instances, American custom and "international", the laity kneel for the Sanctus and the Agnus Dei in all Masses. The new USA SSPX postures are to stand during the Sanctus and the Agnus Dei.
The New Roman Missal by Rev. F.X. Lasance Imprimatur 1937 & 1945 (reprint by Christian Book Club of America 1993):
Page 69 –
Low MassWhen the celebrant enters the sanctuary the laity rise and remain standing till he descends to the foot of the altar to begin Mass. They then kneel and remain so throughout the mass, except during the two gospels, during which they stand. (
Rubricae Generales Massalis, Tit. 17, n. 2.) De Herdt (Vol. I, n. 146) says
this rubric is not perceptive but directive only.
Note – When the last Gospel is that of St. John they make a simple genuflection with the priest at the words “Et Verbum,” etc., then rise and reamin standing until the priest reaches the foot of the altar.
Page 69 – 70
High MassThe rubrics give no direction. Gavantus (Pars I, Tit. 17) and Pouget ( Institutiones Catholicae in modum Catecheseos, Pars 3, Sect. 2, cap7, & 20) say that the people may conform to the rules given for those who are in the choir. Hence the people:
1) Stand – from the time the celebrant enters the sanctuary to the beginning of the Mass
2) Kneel – from the beginning of the mass until the celebrant ascends the altar after the Confiteor.
3) Stand – from the time the celebrant ascends the altar until he goes to the bench after the intonation of the Glora. During the singing of the Kyrie eleison the people sit if the celebrant sits.
4) Sit – while the choir sings the Gloria
5) Stand – from the time the celebrant rises from the bench to the end of the Orations.
6) Sit – from the beginning of the Epistle to the Dominus vobiscuм before the gospel.
7) Stand – during the Gospel and the recitation of the Credo; during the latter they make a genuflection on one knee at Et incarnates est
8) Sit – while the celebrant takes his seat, and whilst the Credo is being sung by the choir, except at the Et incarnatus est… et homo factus est, when they kneel on both knees.
9) Stand – when the celebrant rises to return to the altar and remain standing whiklst he sings Diminus vobiscuм and Oremus.
10) Sit – from the Oremus to the beginning of the Preface – Per omnia saecula saeculorum.
Note: During solemn Mass when incense is used the people stand during the incensing of the congregation. Remain standing for the Preface.
11) Stand – from the beginning of the Preface until the celebrant begins the Sanctus.
12) Kneel – from the Hosanna, etc., until the Per omnia saecula saeculorum before the Pater Noster.
13) Stand – from the Pater Noster until the Agnus Dei.
14) Kneel – from the Agnus Dei until the Dominus Vobiscuм before the Postcommunion.15) Stand – from the Postcommunion and remain standing until the celebrant gives the Blessing.
16) Kneel – during the Blessing
17) Stand – during the Last Gospel and remain standing until the celebrant has left the sanctuary.
Note: When the Last Gospel is that of St. John, the people should make a simple genuflection with the priest at the word ‘Et Verbum,” etc., then rise and remain standing until the celebrant has left the sanctuary.