If you are coming to the Dominican Breviary from the Roman Breviary, as I did back in 2010, one of the biggest differences that you will notice between the two breviaries is that of the office of Prime. The first thing you will notice in the Ordinary of the Dominican breviary is the brevity of the hour of Prime. The next thing you will notice is what appears to be the existence of an other hour, entitled Pretiosa.
There is little information on the internet about this hour, if that is indeed what it is. In the Ordinary, it certainly appears as a distinct hour of the Office. However, in content, it is a mix of prayers and devotions from Prime, as well as additional prayers and devotions. In "The History of the Dominican Liturgy", where he is describing the liturgical practices according to the original Dominican Office, Fr. Bonniwell treats of it briefly in his description of the Office of Lauds:
"While the Laudate was being said, the friar appointed to read the martyrology approached the prior and inquired in a low voice: "Chapter?" If he replied: "No," the martyrology was read in choir; if he said, "After Prime," it was deferred until this time; but if he answered, "Yes," then the reading was to take place in the chapter-room as soon as lauds ends.
Accordingly the friars left the chapel and entered the chapter-room where the martyrology was read and pretiosa was recited. The reading of the martyrology and the recitation of pretiosa in the chapter-room, especially after prime, was the common practice of the monastic Orders in the Middle Ages. On the feasts of nine lessons, Ash Wednesday, and the vigil of Christmas, it was customary to have a sermon after pretiosa."
So it appears as though the practice of splitting off some of what is considered part of Prime (the martyrology and versicles and prayers that go with it) and reciting them separately in choir or in the chapter-room is not unique to the Dominicans. The other thing he notes is that the form of pretiosa found in the oldest Dominican Breviary known to exist (the "breviary-antiphonary found in the archives of the Order), and that found in the 1962 Breviarium S.O.P., are virtually identical:
The only difference between the pretiosa of the breviary-antiphonary and that of today is that the reading from the Gospel or from the Constitutions (not from the Rule of St. Augustine as at present) continued until the officiant gave the signal to stop. Those absent from the office recited the prologue to the Rule of St. Augustine in lace of the reading from the Gospel or the Constitutions.
The rubrics for the Dominican office state that Pretiosa is said after Lauds or after Prime. It begins immediately with the reading from the Martyrology (for the following day of course), followed by the versicle and response:
℣. Pretiósa in conspéctu Dómini.
℟. Mors Sanctórum eius.
℣. Precious in the eyes of the Lord.
℟. Is the death of his saints.
The hour draws its name from this opening prayer.