These are beautiful, finely-wrought and tightly-focused works standing on tense tiptoes at one of western music's most important turning points, peeking both ways. 'The science...

These are beautiful, finely-wrought and tightly-focused works standing on tense tiptoes at one of western music's most important turning points, peeking both ways.

'The science of numbers ought to be preferred as an acquisition before all others… Every art considers reason more honourable than a skill which is practised by the hand or the labour of an artisan,' wrote Boethius, one of music's earliest theorists. For Boethius music was essentially a mathematical discipline – chiefly because the regularities, predictabilities and knowability of mathematics best reflected the Creator's world. Boethius' influence was still strongly felt in the twelfth century when one small area of a relatively small city lead musical development throughout Europe certainly as never before and perhaps as never since: Notre Dame in Paris. Paris Expers Paris ('Paris has no equals') is a splendid CD celebrating the pre-eminence of the Notre Dame school and its achievements from the mid twelfth to mid thirteenth centuries. Two innovations in particular changed the course of music. Firstly harmonically: organum (virtuosic melismatic embellishment of the text in the upper solo voices). Then rhythmical innovations: the imposition of a steady beat on otherwise equal values for notes – as in plainchant. The motet (multiple, usually polyphonic, voices singing multiple texts, sacred and secular) and conductus (usually non-liturgical, or even secular, composition with up to four voices performing original texts) epitomize these innovations.

This pre-eminence in music was led by the two giant composer figures Léonin and Pérotin. They wrote against the background of the great University in Paris with scholars like Abelard, the monastic Mont Ste. Geneviève and the 'Left Bank'. It was amid this vibrant, evolving and questioning atmosphere that neo-Platonism emphasized the relationship of the visible and material with the invisible and immaterial. The essence of Gothic thinking was to bathe in harmony, coherence and unity as Gothic cathedrals bathed in the light of the Divine. It was also inevitable that theories of music should emerge that were specific to its technicalities; theories that no longer sought to explain and explore organum, say, in terms purely of Boethian mathematics. Such a development seems to have conflicted with the prevailing distinctions made by Boethius between the musicus (the speculative theoretician) and the cantor (the operative practitioner). That is, roughly a distinction between composer and performer. The distinction was all the starker since the six solo cantor clerics from the Notre Dame choir known as 'machicoti' memorized and may have improvised their organum. Their role was much more than reproduction of plainchant by this time. That non-written and 'occasional' music-making surely accounts in part for why we do not have more examples of their work. Those that we do are consequently all the more precious.

Paris Expers Paris is a CD with nine examples, ranging in length from under 2½ to nearly 18½ minutes, of the richness of this extraordinary musical development sung most expertly by the six-strong Diabolus in Musica (two tenors, baritone and three bass-baritones) in transcriptions by their director, Antoine Guerber. This is not, though, a purely didactic or dry set of historical curiosities. It's live, spirited and wholly contextualised music. Although it was recorded not in Notre Dame itself but in the Abbey of Fontevraud for the splendid acoustic of its refectory (the venue for this CD) and high dormitory.

Benedicamus Domino and Descendit de celis are three-part organa typical of the melismatic traditions established at Notre Dame by the late twelfth century and are examples of responses by the main choir to the 'specialist', élite machicoti. Sursum corda is a two-part conductus actually itself an exhortation to sing polyphonically – but carefully: 'non discordat vox a corde'! The text contains many musical terms… probably more evidence of its performers' awareness of the important musical changes through with they were passing.

Mundus vergens and Deus misertus are four-part conducti; the former lamenting the current national troubles and political instability in France at the time. The latter was probably introduced by Pérotin, who was the first to write in four parts; it sets Old and New testament texts. Olim sudor herculis is a one-part conductus whose author (for once) we know – Peter of Blois, a noteworthy scholar. Naturas deus regulis, O Maria virginei and Veri floris sub figura are three-part conducti concerning the clergy and Virgin.

These are beautiful, finely-wrought and tightly-focused works standing on tense tiptoes at one of western music's most important turning points, peeking both ways. Diabolus in Musica does the music proud. The centerpiece is the extraordinary and lengthy (over 18 minutes) Descendit de celis. The singing throughout is clear and directed, fluent and without a hint of languor yet Diabolus in Musica sing with an acute awareness of the awe in which this music deserves to be held. Such a balance (between 'edge' and reverence) is surely achieved by the enjoyment and affection which the ensemble brings to their performances, and to the occasion. The singing has a remarkably unified sound to it; the voices blend well and the balance is clean and easy on the ear. Although the weight of the ensemble is towards the lower end of the vocal range, the sound of Diabolus in Musica is reserved and serious rather than sombre.

Attractively-presented in a sturdy 'Digipak' with an informative booklet (parts of it a little hard to read: black on gray) which contains background to everything relevant except, unfortunately, Diabolus in Musica themselves – so see their site and has texts in Latin, French and English. The recording is excellent.

There are other discs of some of the music from this period, certainly by the two great exponents Léonin and Pérotin. But not all, and not all so expertly rendered as here. Paris Expers Paris is a special CD and one to buy and listen to repeatedly with real delight.

Mark Sealey, Classical Net