Above all else, each singer must be thoroughly familiar with the table of Note Values and Their Equivalents in Modern Notation, knowing the precise meaning of each rhythmic marking, alone or in combination, the precise value of each note, the precise meaning of each bar line, and the correct rendition of ornaments (quilisma, descending passing tones, weak beginning notes).

Fundamental Principles

As a rule, short notes come in pairs, and short and long notes are in 1:2 proportion, which creates a steady beat.  The realization of a steady beat (tactus), which neither speeds up nor slows down until the end of the chant, is almost the whole theory of mensuralism or proportional rhythm.  This tactus is the heartbeat of the chant, which propels it forward so that each note and syllable falls into place with vitality, proportion, beauty, ease, and freedom.

Unevenness of singing must not, therefore, be allowed to spoil the sacred chant; no note or neume is to be unduly quickened or retarded; neither may one be negligent and start to sing during a chant (a respond, for example, or any other piece) more slowly than at the beginning.  Another point: Breves must not be slower than is fitting for breves; nor may longs be distorted in erratic haste and made faster than is appropriate for longs.  But just as all breves are short so must all longs be uniformly long, except at the divisions, which must be sung with similar care.  All notes which are long must correspond rhythmically with those which are not long through their proper inherent durations, and any chant must be performed entirely, from one end to the other, according to this same rhythmic scheme.  In chant which is sung quickly this proportion is maintained even though the melody is slowed towards the end . . . .  For the longer values consist of the shorter, and the shorter subsist in the longer, and in such a fashion that one has always twice the duration of the other, neither more or less.  While singing, one choir is always answered by the other in the same tempo, and neither may sing faster or slower.¹ (Commemoratio brevis, 9th/10th cent., tr. Bailey)

So to sing rhythmically means to measure out proportional durations to long and short sounds, not prolonging or shortening more than is required under the conditions, but keeping the sound within the law of scansion, so that the melody may be able to finish in the same tempo with which it began.  But if any time you wish for the sake of variation to change the tempo, i.e. to adopt a slower or a faster pace either near the beginning or towards the end, you must do it in double proportion, i.e. you must change the tempo either into twice as fast or twice as slow . . . .² (Scholia enchiriadis, 9th cent., tr. Murray)

Recitation in Longs

According to the Commemoratio brevis, the psalmody of the Divine Office is sung at twice the tempo of its antiphon, i.e., recitation in shorts, but the gospel canticles (Benedictus, Magnificat, and Nunc dimittis) are sung at the same tempo as their antiphons—recitation in longs:

When the psalm is finished the antiphon is to be slowed by exactly half to its proper tempo.  There is an exception in the case of the Gospel Canticles, which are sung so slowly that their antiphon should follow at the same tempo, and not be further protracted.³

It is evident from the oldest extant manuscript source, Laon 239 (late 9th cent.), that the psalm verses of the Mass at the Introit and Communion likewise involve recitation in longs.⁴  There are examples elsewhere of series of short syllables in the Mass chants, but Introit psalm tones are consistently notated with the reciting tone in longs.  Incidentally, recitation in longs has been retained as an unbroken tradition by the Carthusians, but only for the doxology Gloria Patri, et Filio, et Spiritui Sancto wherever it occurs in the liturgy.

In order to give life, energy, and interest to something that is literally monotonous, we have to exaggerate the accentuation of the text.  The recitation in longs must be declamatory.  As I have said in rehearsal, really dig in.  Sing it like you're proclaiming a sacred text rather than making polite liturgical background music.  When we read through the text in class before singing, always speak as though you are projecting to the back of an amphitheater without a microphone.  Lift up thy voice like a trumpet (Is. 58.2).  Never mumble unintelligibly!

Repercussion

The tristropha [in the introit psalm verses for modes I, III, and VII] should be sung with a “three-fold rapid percussion of the note,” or with a “three-fold rapid blow, like someone knocking with his hand.”⁵ (Aurelian of Réomé, mid 9th cent.)

Every neume is formed of the two motions, arsis and thesis, i.e., upwards [acute accent] and downwards [grave accent], except the repercussed and simple neumes.⁶ (Guido of Arezzo, early 11th cent.)

Guido’s commentator describes neumes which “are double or triple in the repercussion of the same sound.”⁷ (Aribo, late 11th cent.)

We call a virgula or punctum a simple neume; a repercussed neume is one that Berno calls distropha or tristropha.⁸ (John Cotton, early 12th cent.?, quoting Berno of Reichenau, early 11th cent.)

Remarkably, even the fabricators of the Solesmes method themselves admitted as much:

Formerly, the individual elements of this figure [i.e., the strophicus] were characterized by an impulse of the voice. . . .  It would be best to repercuss the apostropha gently and softly.⁹ (Dom André Mocquereau, early 20th cent.)

The above quote is taken from the introduction to the Liber Usualis, where the words referring to the apostropha were rendered in English as “at the beginning of each distropha or tristropha.”  Nearly the same Latin wording was retained in Dom Gajard's revision in the Antiphonale Monasticum, which is given in English in the 1957 Solesmes publication Mass and Vespers as follows:

Formerly these notes were distinguished by a slight impulse or inflection of the voice. . . .  The ideal would be a light repercussion, as it were a fresh layer of sound, on each apostropha.

Compare what can now be considered the mainstream semiological approach:

From the primitive Gregorian notation it is evident that two or more notes on the same pitch and the same syllable are never to be conjoined into one sound: hence each of the notes in the strophicus, trigon, and every other grouping of this sort is to be repercussed.  In passing from one word to another on the same vowel and on the same pitch, a repercussion is made.¹⁰ (Liber Hymnarius, 1983; tr. Peter Jeffery)

These repercussions are more analogous to what is known in modern music as portato, mezzo-staccato, or articulated legato than to pure staccato.  Musicians sometimes need to be reminded that staccato itself simply means detached, not as short as possible (staccatissimo), and not accented.  It must be kept in mind that the repercussions in chant occur under the umbrella of an overarching legato line.  They should be executed tastefully and gracefully, without exaggeration, with only the amount of detachment necessary for the notes to be heard distinctly, which is relative to the acoustics of the room.  I recommend simply using a light h sound between notes.  In the execution of repercussions there must be absolutely no movement of the jaw or tongue and no change in the shape of the mouth, lest the vowel be modified or, worse still, something like ayaya or owowo be pronounced—a caricature of the Latin text!

1. Inaequalitas ergo cantionis cantica sacra non viciet, non per momenta neuma quaelibet aut sonus indecenter protendatur aut contrahatur, non per incuriam in uno cantu verbi gratia responsorii vel ceterorum segnius quam prius protrahi incipiatur.  Item brevia quaeque impeditiosiora non sint quam conveniat brevibus, nec longa inaequalitate lubrica festinantius labanturquam conveniat longis.  Verum omnia longa aequaliter longa [sicut] brevium sit par brevitas, exceptis distinctionibus quae [nihilominus] simili cautela in cantu observandae sunt.  Omnia quae diu ad ea quae non diu legitimis inter se morulis numerose concurrant et cantus quilibet totus eodem celeritatis tenore a fine usque ad finem peregatur.  Hae tamen ratione servata dum in cantu qui raptim canitur, circa finem . . . longiori mora melos protendendum est. . . . ut pro modo brevitatis prolixitas prolongetur, et secundum moras longitudinis momenta formentur brevia, ut nec maiore nee minore sed semper unum alterum duplo superet.  Dum canente quolibet respondetur ab alio unum morositas servent utrique modum, nee unus altero impeditiosius aut celerius canet.

2. Sic itaque numerose est canere, longis brevibusque sonis ratas morulas metiri, nec per loca protrahere vel contrahere magis quam oportet, sed infra scandendi, legem vocem continere, ut possit melum ea finiri mora qua cepit.  Verum si aliquotiens causa variationis mutare moram velis, id est circa initium aut finem protensiorem vel incitatiorem cursum facere, duplo id feceris, id est ut productam moram in duplo correptiore seu correptam immutes duplo longiore . . . .

3. Repetitio antiphonarum quae in fine versuum inter captandum fit eadem qua psalmus celeritate percurrat, porro finito psalmo legitima productione protendatur duplo dumtaxat' longius.  Excepto dum cantica evangelica sic morose psalluntur, ut non longiori, sed eadem morositate antiphonam subsequi oporteat.

4. See also the translator's note on p. 41 of Jan van Biezen's “The Rhythm of Gregorian Chant,” tr. Kevin M. Rooney in Rhythm, Meter and Tempo in Gregorian Chant.

5. Terna gratulabitur vocis percussione . . . trinum, ad instar manus verberantis, facias celerem ictum.

6. Motus vocum . . . fit arsi et thesi, id est, elevatione et depositione: quorum gemino motu, id est arsis et thesis, omnis neuma formatur, preter repercusse aut simplices.

7. . . . cum duplices aut triplices in eiusdem sunt soni repercussione.

8. Simplicem autem neumam dicimus virgulam vel punctum: repercussam vero, quam Berno distropham vel tristropham vocat.

9. Singula hujus figurae elementa olim nonnulla vocis reparatione discernebantur. . . .  Optimum quidem esset apostropham leniter molliterque repercutere.

10. E notatione gregoriana primitiva constat duas aut plures notulas eiusdem gradus in eadem syllaba numquam coniugi in unum sonum: unde unaquæque notularum in strophicis, in trigonis aut in omni alio huiusmodi concursu, repercutiatur.  Cum a verbo ad alium in eadem vocali transitur, si in eodem gradu, fit repercussio.