This form, which developed out of the polyphony of the organumand was first of all employed in the worship of the Church, very soon grew away from its origins and became the most important single form of musical expression, whether ecclesiastical or secular. Indeed, although the academic composers of the period were 'clerks' in the medieval sense of the word and therefore in the last resort 'church composers', the term was valid only in the most general sense. During the 14th century especially, the finest products of academic music were, with one or two outstanding exceptions, in the secular field. Nevertheless, the distinctions between 'religious' and 'secular', were far less clear than they were to become in later ages -- in his famous bull of 1326, Pope John XXII fulminated against the introduction of profane words into the church motetus.Furthermore, the composers of the period were inevitably the heirs of the ecclesiastical polyphonic tradition and not of the tradition of the troubadours, the true ancestors of Western secular art music.
One of the earliest procedures for varying the music of the liturgy was the trope. New passages of music or text were interpolated into the existing musical structure, and from this early practice a new form, the sequence, had developed. When this same principle of interpolation was applied not to the monody of the plainsong but to the polyphonic structure of organum,the result was the polyphonic motet, which emerges, in France, in the second half of the 13th century. The plainsong tenor of the earlier organumwith vocalization was divided into a number of distinct musical units which corresponded to words or phrases of the text. Such melodic units or cells were set to phrases such as 'In veritate,' 'Mane prima,' or even short clauses. The divisions set up by the tenorwere observed by the voces organaleswith each of the words constituting a section, called a clausula.These clausulae,which were in effect musical fragments, formed the vital element in the birth of the motet. It became increasingly common for musicians to vary the organain their cathedral repertory by interpolating clausulae from one organumin another. When this was done, the original text of the clausula,which would of course be out of place in its new setting, was dispensed with, and, instead, the interpolated section of music would be performed as a vocalization. In this way the organumcould be musically varied without, however, losing its original character. The next stage of the development towards the motet was the addition of words to the upper part of the clausula,and this part soon became known as the motetus(from mot.'word'). As often as not this new text would be a commentary in the vernacular French on the sense of the Latin tenorand the basic elements of a new form were now present, a form which soon achieved an independent existence. The final stage in the evolution of the full motet of the 14th century was the addition of a third part, triplum,to the tenorand motetus.The result was a three-part work consisting of a slow-moving lower part, the original tenorsung to its original short Latin phrase; a faster moving middle part with a French or Latin text and above that a still more florid part which, in its turn, was often set to another text which might also be in French or Latin. Although the two lines of text might be not only different words but even a different language, the result was diversity rather than disorder since the two texts were related to one another in sense.
After first being an integral part of the organum,the polyphonicclausulaecame to be performed in isolation during the service, although we do not know in what circumstances. In fact the new form of the motet had difficulty in finding its place in liturgy, for hardly had it been created than it left the Church and won an unprecedented popularity, its success contributing to the decline of the organumitself. Composers more and more abandoned the great religious fresco to apply themselves to this finely worked miniature. Using a relatively limited number of canti,they wrote new voces organaleswith new words. Soon the practice grew up of giving the upper voices French texts related less and less obviously to the Latin tag of the tenor; thus a tenor taken from a passage in praise of the Virgin would have superimposed above it a French love song. From being religious, the motet became merely edifying and by the end of the 13th century had become an exclusively profane form with only the tenor-- which might well now be wordless, being performed on an instrument -- witnessing to its liturgic origin.
The composite form of the medieval motet presents special problems to the modem listener. Not only do the words seem at first sight prejudicial to the unity of the music and the relationship between the parts obscure, but our inevitable tendency to think in terms of vertical harmony rather than to follow the individual lines is a further handicap. As we have said already, these parts are by no means strangers to each other. The sense of each one is related to the idea expressed by the tenor,and until the very end of the 13th century this relationship, no matter how slight, was to survive. It is now often difficult to discover this relationship, especially when the composition depends on refined symbolic or allegorical relationships of meaning between the tenorand the other voices. The 13th-century motet, however, was ideally suited to beguile away the leisure hours of a refined society which delighted in musical and literary games, and may well have been performed by amateurs for their own entertainment rather than in a concert. It has also been suggested that in concert performance each of the voices may have been sung in turn while the others were replaced by instruments, all these texts being sung simultaneously only as a finale to the performance. In conclusion it may be observed that the simultaneous rendering of different, yet related, texts in this way is in fact met with often enough in 18th- and 19thcentury opera.