The period from 900 to 1300 has been labelled the Silver Age of chant for four musical achievements. First, most of the Ordinaries were composed in this period. Only a few fragments of Ordinaries from the Golden Age remain, their neumatic (one note per syllable) style distinguishing them from later Ordinaries, which are more likely to lavish multiple notes on a single syllable (the melismatic style).

Second, the Silver Age gave birth to hymns and Propers still sung, as well as to a portfolio of music (tropes, sequences and conducti) closed to Catholics since the Council of Trent (1563) pruned back the liturgy. Sequences and tropes may have started life as a singer's memory aid. In the introduction to a collection of his sequences, Notker Balbalus (the Stammerer, 840-912), a monk of the St Gall monastery in Switzerland, tells the story of a French monk who sought refuge at St Gall in 862, following the Norman sacking of his monastery at Jumieges, in Normandy. Notker noticed that the monk's antiphonary (chant book) showed new words fitted to melodies that St Gall sang on a single syllable. Struck by the notion that words might help him to memorise the long wordless melodies, Notker revised the St Gall chants accordingly. The melody lines with new words were called tropes.

Most trope texts commented on the meaning or implications of the original chant text, a type of gloss common in medieval law and literature. Although liberally applied to Mass chants (even the Epistles!), tropes are most strongly associated with the Kyrie. The titles given to Ordinary Masses (for example Missa Dominator Deus, Missa Cunctipotens Genitor Deus) tend to be drawn from the opening words of exiled Kyrie tropes.

Sequences were sophisticated tropes, exclusive to the Alleluia. Instead of just filling in the jubilus that long burst of song on the last syllable of the word "Alleluia" sequence composers were inspired to trope the whole Alleluia! Doubling each line of the Alleluia created a series of couplets, to which the composers added freshly created text. Being born of prose rather than verse, the couplets in early sequences were irregular lengths (see Victimae Paschali Laudes, page 14). But later sequence texts were metrical poetry, with a melody adapted from the Alleluia or from a popular tune, and rhymed couplets of equal lengths. St Thomas Aquinas, for example, set his sequence for the new feast of Corpus Christi to a popular tune by Adam de la Halle.  The sequence was soon accepted as an independent chant, but retained its position directly after the Alleluia.

One of the most popular of all forms of liturgical music, sequences became a standard part of the Mass. There were sequences for the Season, for the Saints, sequences unique to a region. Their popularity did not preserve them from ruthless pruning at Trent, which prohibited all but four: Victimae Paschali Laudes, Dies Irae (attrib. Thomas of Celano, d. 1250), Aquinas' Lauda Sion Salvatorem, and Veni Sancte Spiritus. Stabat Mater (attrib. Jacopone da Todi, d. 1304) was authorised in 1727. Tropes were also prohibited at Trent, a ruling which unfortunately rendered unuseable many early polyphonic Masses that had been troped.

Conducti were Latin songs marked chiefly by their liberation from any standard characteristic. They might be rhymed, rhythmic, metrical - or not; some resembled hymns (in stanzas), some sequences (in couplets), some had refrains, others used different music for every line, like 16th century madrigals (a technique called 'through-composed'). The only regularities were that the lines tended to be the same length and, like hymns composed in this period (for example, Gloria Laus et Honor), the music was more melismatic than syllabic - possibly an attempt to harmonise stylistically with the other chants in the Mass. Conducti were used during liturgical processions or whenever the priest or other parties were 'conducted' from place to place. Conducti were quickly adopted by the secular sphere, and appear in medieval dramas, at the entry and exit of characters.

Famous composers from the Silver Age, besides those already mentioned, were: Adam of St Victor (d. 1192), writing from the Abbey of St Victor just outside Paris; Hermannus Contractus (the Cripple, 1031-54), from the monastery at Reichenau, who produced a Salve Regina as well as the Alma Redemptoris Mater (popular enough in the Middle Ages to rate a mention in Chaucer's The Prioress's Tale); Abelard (1079-1142), who composed a book of hymns for the use of Heloise's nuns and his own monks; and Hildegarde of Bingen, also writing for her own nuns. St Thomas Aquinas wrote not only Lauda Sion (adapting music from a popular song by Adam de la Halle) but also the hymn Anima Christi, still familiar to Catholics.

Chants were differentiated not only by form (sequence, trope, conductus, hymn) and the number of notes per syllable, but also by style. Hildegarde's smooth flow of notes, with relatively few pauses or jumps to notes much higher or lower, contrasts with the Gregorian style of chant, which is marked by comparatively short, repeated melodic phrases and by frequent fourths. The different religious orders also fostered different styles: Benedictine chants were more elaborate than Dominican chants and generally sung at a higher pitch than Carthusian chant. Then there were melodic and rhythmic divergences - a note added here or held longer there - in the chant sung at different monasteries, especially at musical centres like St Gall, St Martial of Limoges in France or Reichenau in Germany.

We know of such differences partly through musical notation, the third great contribution of the Silver Age. Some form of notation probably existed from early medieval centuries - it is hard to imagine how Pope Gregory could codify the chant without writing it down or how the vast Gregorian repertoire could be faithfully transmitted to distant lands solely by memory. Moreover, a manuscript from the Council of Cloveshoe (Glasgow) in 747 refers to a sample of 'the method of chanting' received in writing from Rome. However the earliest surviving manuscripts with notated music date from the 9th century. That notation indicates only the general shape and grouping of the melodies (up or down, fast or slow, notes to be sung as a group), so singers must have known the year's chant by heart and simply used the notation (called neumes) as a reminder. The notation differed from monastery to monastery (and scribe to scribe!). Some of the best know varieties are Sangallian, Aquitanian, French, Norman, Beneventan, PaleoFrankish, Messine and Gothic.

This type of notation lasted until the 14th century in some countries. But by the High Middle Ages, the experiments of dissatisfied music theorists had produced a more precise form of notation. Early staff notation suspended neumes on invisible wires around a real or imaginary line. The distance between each note indicated the interval, and the line was designated as F and coloured red. Later, another line (yellow or green) was added to indicate C, and then another two, to form the standard chant four-line stave. By the 12th century, Italian, French, Spanish and English monasteries commonly used staff notation. A century later, neumes had developed into the square shaped notes we use for chant today.

The fourth contribution of the Silver Age is, like habited nuns, more often found on film than in the cloister. Organum was Christendom's first taste of harmony and chant's last flourish before the attention of the medieval world wandered on to polyphony. Organum’s open fifths were, indeed, the doorway to polyphony. Organum replicates a chant note for note (or punctum contra punctum, from which derives the term 'counterpoint'), but a fifth or a fourth above or below. The effect is a hollow, brassy harmony, prone to sneaking unnoticed into the exercises of music students. Although condemned by modern musicianship, to ears that had never heard harmony the sound of parallel fifths enriched the plain chant, making organum specially suited to feasts and celebrations.

The last rapid steps to polyphony were taken with the development of rhythmic modes and notation (metre); the addition of independent text to each organum part; and the independence of the harmony parts from the text, rhythm and melodic shape of the plain chant. These developments made harmony parts no longer an accompaniment to chant but melodic lines in their own right. Central to these developments were the Monastery of St Martial at Limoges and the choir of the newly built Cathedral of Notre Dame in Paris.

With the development of polyphony began the Church's continuing battle to maintain or restore chant to its central place in the liturgy. From the 13th century on, chant sank into a downward spiral of disuse and misinterpretation, not halted until the opening of the 20th century.monies towards the sort of beauty found in shifting shades of light or the ripening of seasons.