Rankin:

The famous Gradual Laon Bibliothèque municipale 239 may have been made within fifty years [ca. 880] of the German book [Augsburg Stadtsarchiv Kloster Holzen MüB Literalien 104, ca. 830].  Laon 239 is now the earliest extant fully notated and almost complete gradual: on palaeographical grounds both Bischoff and Contreni have dated it in the ninth century. (Susan Rankin, Writing Sounds in Carolingian Europe, p. 67; in a footnote on p. 86, she adds, “It is unclear from where or on what basis the dating to 930 emerged.”)

Vollaerts:

Notes on the value of Laon 239
  1. By distinguishing longs and shorts in syllabic passages, only Laon 239 has preserved intact a primitive tradition.  Only this MS has saved from mutilation what has been dispersed over several other MSS as incomplete fragments of a crumbling tradition.  Only in Laon 239 has this tradition remained intact.
  Since Laon as a whole, corresponds completely and positively with all the other MSS together, it would be patently absurd to suppose that this MS does not maintain the tradition.
  2. It is evident that for its ‘noting’ of syllabic passages, Laon is superior to the other complete documents (we possess only eight Nonantolian pages); Laon is accurate, and at the same time the only MS which is accurate.
  Shortly, it will be proved that this evaluation applies also to the manner in which the Laon MS clearly indicates a long sound by means of its virga, and in contrast, a short sound by its point. (Jan W. A. Vollaerts, Rhythmic Proportions in Early Medieval Ecclesiastical Chant, 2nd ed., pp. 44–45)

Murray:

A comparison of the Metz notation with that of St Gall reveals important differences.  The St Gall notation, by employing symbols derived from the acute and grave accents, is mainly concerned to show the rise and fall of the melody by the shapes of the neums.  Thus its punctum or point (being derived from the grave accent) always indicates a relatively lower note (i.e., a note lower than either the previous note or the subsequent one).  The Metz notation, by way of contrast, reveals greater concern to differentiate between the lengths of the notes.  Consequently a Metz punctum is used quite as frequently for high notes as for low ones—provided they are short.  A similar contrast is found in the different uses of the virga in the two notations.  In St Gall the virga, being an acute accent, always represents a relatively higher note.  The Metz virga, on the other hand, is only used for higher notes when they are long.  There are some exceptions to this rule, but they are comparatively rare. (Gregory Murray, Gregorian Chant according to the Manuscripts, p. 13)

Van Biezen:

The main characteristics of the Laon style of neumes are the fundamental disjointedness between non-cursive neume elements and the conjoining where possible of cursive neume elements.  Our perception moreover has the tendency to imagine a short note or a series of short notes as being associated with or moving toward a subsequent long note.  The neumes of Laon are therefore especially adequate for rendering short versus long notes.  It is also striking that the last note in a series of connected neume elements can be either short or long. (Jan van Biezen, “The Rhythm of Gregorian Chant,” tr. Kevin M. Rooney in Rhythm, Meter and Tempo in Gregorian Chant, pp. 25–26)

Floros:

One must consider first of all that the codex Laon 239, the main representative of the Metz tradition, belongs among the oldest surviving sources and it has been proven to be an extremely correct and carefully written manuscript.  The assumption of errors on the part of the copyist is quite absurd. . . .  In conclusion it can be consequently stated that Dom Mocquereau’s thesis about the “corruption” of the Metz tradition is null and void.  The investigation of the surviving sources of Metz notation reveals on the contrary that the high regard that was accorded the “Metz chant school” in that 9th and 10th century is justified. (Constantin Floros, The Origins of Western Notation, tr. Neil Moran, pp. 291–2)

Cardine et al.:

Recent studies have established its particular value in the field of rhythm.  L is frequently compared here with the St. Gall MSS and it is this which has formed the principal basis of our semiological study. (Eugène Cardine, Godehard Joppich, and Rupert Fischer, Gregorian Semiology, tr. Robert M. Fowells, p. 10)

Heckenlively:

The value of the long neums is revealed to us by the comparison with the Saint-Gall manuscripts.  In fact the ordinary Messinian signs correspond exactly with the ordinary Sangallian signs.  There are thousands of examples of this concordance between the two schools. . . .  It was due primarily to the abandonment of the rhythmic signs, that the interpretation of the chant was lost.  But in spite of all this, the accordance between the two schools, Metz and Saint-Gall, in the matter of rhythm, is astonishing : A decisive proof that the same rhythmic laws, even to the finest details, were imposed from the beginning, on the entire Catholic world. (Lura Heckenlively, The Fundamentals of Gregorian Chant, pp. 159 & 288)

Blackley:

It was said that neumes or linear shapes marked above the texts in chant manuscripts of the late 9th–10th centuries were hand-signs, and this I trustingly accepted, though wanting for practical understanding.  While in the Christian Brothers I’d conducted equalist-rhythm chants for all Sundays and for major feasts using cheironomy, linear phrases the hand traces in air.  Later . . . it had dawned on me that cheironomy was out, that I had in fact only to conduct each of the very neume shapes themselves, [which] tell the conductor what to do with his or her hand.  Nothing could be more clear. . . .  When chant is sung carefully in equalist rhythm, it can be lovely.  But when it is sung correctly from the heart in proportional rhythm, it is shapely and beautiful.  Notations other than Metz, while in remarkable agreement with it, offer less clear visual directives for the conductor, and these help make Metz and Laon 239 so important. (R. John Blackley, Laon 239: Chant Transcriptions in Proportional Rhythm, English & Latin, p. 481)

Wagner:

The Metz song-school, the eldest daughter of the Roman Schola Cantorum in the kingdom of the Franks, remained, even after Charles' death, the most influential among her numerous sisters.  It was held in high esteem until the 12th century, and was always regarded as the faithful guardian of the Roman chant.  In the 9th century the Metz chant held this position, and had the reputation of being far superior to all the other Churches in France and Germany, as John the Deacon expressly intimates : ‘Denique usque hodie quantum Romano cantu Metensis cedit, tantum Metensi ecclesiae cedere Gallicanarum ecclesiarum Germanarumque cantus, ab his, qui meram veritatem diligunt, comprobatur' [“In short, to this day, as much as the Messine is inferior to the Roman chant, so much is the chant of the Gallican and German churches inferior to that of the church of Metz, which is acknowledged by those who love pure truth.”] (Patr. Lat. lxxv, 91).  This remark of Gregory's biographer has a polemical ring, and is directed against a tendency to belittle the merits of the Metz song-school in favour of another, which can only be S. Gall.  This view is confirmed by the anonymous biographer of Charles the Great, who also wrote towards the end of the 9th century; according to him, throughout the regions of France in which Latin was spoken, in place of the term Ecclesiastica Cantilena the term Cantilena Metensis was used (Patr. Lat. xcviij, 1378).  (Vita Carol. I, 11).  This evidence is all the more above suspicion as it comes from a monk of S. Gall.  It seems in fact that the activity of the Song-school of Metz has hitherto been greatly underrated. (Peter Wagner, Introduction to the Gregorian Melodies: A Handbook of Plainsong, part 1, tr. Agnes Orme & E. G. P. Wyatt, pp. 214–5)

The Laon municipal library website dates Codex Laudunensis 239 to the ninth century and includes an article by Marie-Noëlle Colette dating it to the late ninth or early tenth century.  Regardless of its antiquity, the value of Laon 239 cannot be overstated.

On April 25, 2024, Dominique Gatté announced that he had identified a fragment posted to the blog of the libraries of Northwestern University as a page of Laon 239, another fragment of which was published in vol. 10 of the Paléographie musicale in 1910.  The page shows seven alleluia verses, including those for the fourth Sunday after Easter and the last Sunday after Pentecost, plus parts of two others.