How one who does not come to the daily prayer before the end of the first Psalm is not allowed to enter the Oratory; but at Nocturnes a late arrival up to the end of the second Psalm can be overlooked.
But one who at Tierce, Sext, or None has not come to prayer before the Psalm is begun and finished does not venture further to enter the Oratory nor to join himself to those singing the Psalms; but, standing outside, he awaits the breaking-up of the congregation, and while they are all coming out does penance lying on the ground, and obtains absolution for his carelessness and lateness, knowing that he can in no other way expiate the fault of his sloth, nor can ever be admitted to the service which will follow three hours later, unless he has been quick to make satisfaction at once for his present negligence by the help of true humility. But in the nocturnal assemblies a late arrival up to the second Psalm is allowed, provided that before the Psalm is finished and the brethren bow down in prayer he makes haste to take his place in the congregation and join them; but he will most certainly be subjected to the same blame and penance which we mentioned before if he has delayed ever so little beyond the hour permitted for a late arrival.
Source: Institutes (New Advent)