Anyone who wishes to speak about the Tridentine rite must first clarify what meaning he exactly attributes to this term. Essentially the Trident rite is not an original and independent liturgy but a variant of the centuries-old Roman liturgy. Compared with this centuries-old tradition the first points that strike the eye are where the Tridentinum seems to be independent and different from everything else. In this case it will be declared to be a new branch of the Roman liturgy created 400 years ago. On the other hand, if you compare the Trident rite with liturgies other than the Roman one (e.g. with 17th—18th century “reformed” liturgies or with their spiritual offspring, the Bugnini liturgy) you will find that there are no important differences between the Tridentine usage and the main stream of the at least 1500-year-old Roman liturgy. In this respect the Tridentine rite represents the ancient Roman liturgy itself and not a 400-year-old custom.
Evidence on the liturgy of the first Christian centuries is sparse, and certainly not enough to justify any practical arrangement of the liturgy. Authentic sources survive sporadically from the 6th century onwards and in complete form from the 8th-9th centuries on. The liturgical arrangement as it is recorded in the 8th- to 10th-century sources could be traced back with careful consideration to the end of the 5th century at the latest. (Just think of the structural identity of the Office in the 9th—10th-century liturgical books on the one hand and in the Rule of St. Benedict on the other.) The main features of this liturgical usage were the same as those of the Roman church as followed — with slight differences — by the dioceses, provinces, religious orders up to 1970. There was no universal Roman liturgy that showed deviations depending on the location and the times of its use, in fact, these variants represented in their integrity the common essence of the Roman liturgy. The differences of the individual liturgies are historically extremely interesting and the details very clear if we examine them closely. However, the moment we look at them from the perspective of our time, they appear to constitute a uniform, characteristic liturgical family separated not only from the rite of the Eastern churches but from the liturgy of the other Latin rites (e.g. Mozarabic, Ambrosian, etc.), too. Unity and difference within this “Roman” liturgy (or liturgies) is not accidental, and the number and importance of the common features are greater than that of differences.
Laszlo Dobszay, The Bugnini-Liturgy and the Reform of the Reform (2003)