Semiology is the study of the signs, that is, the earliest neumes notably from the Saint-Gall and Laon manuscripts (and related manuscripts, like from Einsiedeln). It is strongly associated with Dom Eugène Cardine of Solesmes who wrote a sort of academic guide to it (helpfully called Gregorian Semiology. Solesmes, at least in the 1980s and through the early 2010s, informed its chanting via his own understanding of what this meant for music, although in some ways it didn’t change much from the older practices, even when they officially removed the rhythmic signs from the new Antiphonale Monasticum of the 2000s.

Mensuralism means that the music is measured. There are a couple of ways that I think fall under this: post-1600 chant in much of Europe is mensuralist (I think if you’re considering Solesmes anyway). It could be proportionalist, and Patrick Williams (@FSSPMusic here) would define that as the notes being in a 2:1 ratio of long to short based on his understanding of the treatises and adiastematic neumes. This requires a semiological approach because the ratio is informed by the notation particularly the Laon neumes; in contrast, Cardinian practice tends to prefer Swiss manuscripts. I think that it’s helpful to separate out using the adiastematic neumes; many European scholars and chanters consider their work semiologically-based (as I understand it) without being strictly Cardinian…although there is certainly overlap. The new Nocturnale Romanum winds up taking that approach and uses more rhythmic signs than Solesmes historically did as a result.

Then someone like Mgr Turco is not quite as strict about proportions, but he’s still using the manuscripts to inform the melody and rhythm, as well as treatises etc.

Dom Gregory Murray went back and forth on Solesmes (he also switched rather violently to prefer the English Mass, I’m told), but he also wrote a lot on proportionalism and specifically against the equalist approach of Solesmes. I would agree that for many of us the traditional approach is Solesmes, abstracting from judging that for a minute.

It’s too bad Mgr Turco’s recordings are hard to find due to copyright issues I guess. Patrick once shared a link to the gradual of this Sunday (Laetatus Sum) that is semiological but not quite strictly proportional.

The impulse to compare is a good one; Dom Mocquereau and Dom Pothier did not record the same chants but they sang in a contrasting (although very much related style) at the beginning of the twentieth century at a major event in Rome. @Charles_Weaver made us sing a chant in a contrasting style: I took the easy way out and did Solesmes and Vatican style. Others did semiological (Cardinian) approaches, and in reality I inform my own chant with the living tradition of the abbeys and occasional checking against the Graduale Triplex; it’s semiology lite. I don’t know if anyone sang a proportional chant. It’s probably the most foreign to us even though Patrick’s argument for it in part is because every other kind of music that we sing is measured!

May I also suggest that the terrain or context in which these distinctions are effectively applied and encountered are chants that are sung by scholas/choirs, rather than the Ordinary chants and responsories that are also sung by faithful in the pews?

In practice that’s the case Liam but I hesitate to say that it’s strictly true; as much as I also hesitate to speak for others. That is I get the sense that there are proportionalism advocates who would do that to the Ordinary as well.

They don’t care!