Looking at the Gradual and Alleluia texts a few Sundays back during Mass, I found myself thinking, rather bored, how alike all the texts were; I wondered idly why the Church hadn't used some of the more searing phrases from the psalms: "I have spent my years like a sigh" - phrases which toll with a pang in our hearts.

After a moment's comparison, however, I realized that the Church had chosen texts whose subject was God, not human experience; and I felt rather embarrassed. Long ago, somebody more godly was sufficiently inspired by these texts to set them to music: a Levite in post-exilic Jerusalem; or a Christian, writing from some cold, war-damaged monastery. Most likely, both played a part.

For the chant used in the Mass today is a child of the chant that Christ would have heard in the Temple and synagogues. The early Christians neither composed commissions to revise the liturgy, nor commissioned composers to recast the chant. They brought to the Christian liturgy the prayers and chant of the Jewish liturgy, and the style of music changed little during Christianity's catacomb years. Etheria, a Spanish nun on pilgrimage to the holy places in about 385, mentions hymns, psalms, responsories and antiphons as part of the Easter liturgy at Jerusalem, the first three being forms familiar from Jewish liturgy.

Antiphons are short pieces of prose set to music and placed like sentinels at either end of the psalm. Today, the bulk of the Latin Office is antiphonal psalm singing, and, at Mass, you hear antiphonal psalms when the priest enters (the Introit); during the Offertory (the Offertorio) - although the music for the antiphon has been embroidered to such length and complexity that the psalm itself is usually omitted; and during the reception of Communion (the Communio) - again, without the psalm.

Following the legalisation of Christianity in 313, different forms and flavours of chant began to develop by region. Roman Spain produced Mozarabic chant, whose title refers to the Moorish rule over Spanish Christians after the invasion of 711. In fact, the chant was composed and complete by the 7th Century, and altered little thereafter. From Milan came Ambrosian chant, named in honour of St Ambrose; from Gaul, or what is now France, Gallican chant; from Rome, Old Roman and Gregorian; from England, the Sarum; from the Church in the East, Syrian, Byzantine, Coptic, Ethiopian and Armenian. Some of these chants were suppressed by Roman pontiffs striving to establish a unified liturgy and music for the Church. Others were abandoned when the region resolved to adopt what it considered a superior chant or liturgy. By these paths Gregorian chant came to dominate liturgical music in the West by the 8th Century.

The one fact almost invariably known about Gregorian chant is that Pope Gregory had something to do with it. In fact, a number of musical popes before him had contributed to the development of chant in Rome, forming chant schools, founding monasteries to preserve and maintain the chant or even composing chant. Pope St Gregory the Great (540-604), however, gathered together the different forms of chant; chose, adapted and ordered them; and had them transcribed into an Antiphonary, which in later centuries travelled long distances to bring Gregorian chant to other countries. He not only organised the chant but also took a firm hand in the chant school. "There today", wrote John the Deacon, a 9th century biographer of Gregory, "is still shown the couch on which he reposed while giving his singing lessons; and the whip with which he threatened the boys is still preserved and venerated as a relic." Despite the famous medieval picture of the Holy Spirit singing melodies into Gregory's ear while he attentively transcribed them, we don't know whether he composed any chant.

If Gregory did not write them, who did? Unfortunately, for the most part, the composers are unknown. Scribes sometimes attributed hymns to mediaevally renowned poets: St Ambrose (d.c. 397), Aurelius Prudentius (d.c. 405), Caelius Sedulius (d.c. 450), Venantius Fortunatus (d.c. 610), St Isodore of Seville (d. 636). But these writers may have set their lyrics to popular tunes, rather than compose original melodies.

The chants of the Mass and Office are largely anonymous. Most Proper chants - those intended for a specific Sunday or feast day (Introit, Gradual, Alleluia, Offertorio and Communio) - were composed between the 5th and 8th centuries; although the composition of new Propers, to accompany new feast days, continued until the post-Vatican II period. The Ordinary chants - those common to every Mass (Kyrie, Gloria, Credo, Sanctus and Agnus Dei) - that we sing today were composed in the second wind of chant composition, from the 9th to the 12th century. As in Judaism, the anonymous composers were probably clergy, religious or cantors from the Church's chant schools.

The anonymity of chant composers may be a product of inherited beliefs about music and the techniques of composition these generated. Greek philosophies about the cosmic origin of music and its ethical effect on man, handed on to both medieval West and Byzantine East, resulted in strict principles on combining notes and a reverence for existing melodies, which, in the words of Pseudo-Dionysius (c. 500), "transmit to us the echo of the divine hymns which are sung in heaven and can only be passed on through divinely inspired men". Where modern art prizes originality and, in pursuit of individual expression, rejects prohibitions, medieval composers were expected to combine and adapt the divine ingredients, not to replace them. The medieval art of composition therefore consisted of embroidering melody, patching together patterns from different sources to form a new chant (a process called centonization, also the central technique in Byzantine hymnology), or adapting chants to new texts. The Lenten Tracts are one familiar example of a single melody adapted to multiple texts.

Whether by instinct or by obedience to the Church principle that music should express text, the medieval composers also brought to the text an intense responsiveness that produced word-painting or deeply atmospheric chants. The outcome was a repertoire of melodies built with such an eye to balance and beauty, such sensitivity to the text, that they were mined for musical material for ten to fifteen centuries afterwards. From these melodies, too, were developed the basic rules for writing melodies still taught in musicianship classes today.

While discussing litanies, a friend once remarked that she had never heard a tune she liked. One of the difficulties modern ears have with chant is the love of a catchy tune: look at pop music, folk songs, country music, hymns... Even in classical music, don't you like best the bits you can hum? Another difficulty is our desire for harmony, the vertical richness of many notes sounded simultaneously. In chant, there is no harmony and rarely a catchy tune. Chant is pure melody, and its richness lies in subtle variation of the melody. Only ears accustomed to listening horizontally readily perceive its patterns and changes. The modern listener who wishes to appreciate chant must switch his desires to a different channel, turning from the satisfaction of tunes and harmonies towards the sort of beauty found in shifting shades of light or the ripening of seasons.