At least in Western culture, the idea is as old as the organ itself. The pipe organ was introduced into the Western Church at some point after about 750 when the Byzantine Emperor Constantine V presented a small instrument to Pépin le Bref, King of the Franks. Pépin's son, Charlemagne, eventually acquired that or a similar instrument and placed it in the original Carolingian octagon of what is now the Cathedral in Aachen, Germany.What to do with such a device? As had been the case anciently, the organ was well suited for festive events - the Roman arena, in places of meeting and enjoyment in Persia and Syria, and throughout Byzantium. It would have been inevitable that a pipe organ standing in a Christian church would thus find its role in ritual, but it did so through a key concept. Recalling that the Church recognized that liturgical texts were the exclusive domain of the Sacred Ministers - priests, deacons, subdeacons, lectors- even the singing of a schola or a choir only "stood in." Roman Catholics of a certain age may recall the vestigial evidence at High Mass before 1965 at which priests would recite soto voce even the texts of the Mass Ordinary simultaneously being sung by a choir. Speech (especially that of a veteran cleric!) moves far quicker than song. Thus would congregations witness celebrants returning to their sedillae while the choir soldiered on to its final cadence still minutes away! Those who contended that fancy music in church was mere entertainment had a point. If voices could then "borrow" these liturgical texts from the "official utterance" by a cleric, it would have been but a short jump to have early instrumental music coupled to this "entertainment." Instruments, particularly the organ, could do what voices could not in evolving and more complex musical structures. They could sustain tone theoretically endlessly and relieve singers. So, a practice dubbed alternatim grew wherein instrumental versets punctuated strains of chant usually with organ iterations of the melody devoid of text. No matter. The celebrant had already recited these, and all was canonically legal. Pope Pius X's 1903 Moto proprio on church music specifically outlawed the practice (with varying success as alternatim persists in some important centers. Anyone who has heard a glorious Te Deum or Magnificat at the Cathedral of Notre Dame in Paris would attest to the thrill of hearing the Grand Orgue at the building's west end "declaim" the text between verses by the maitrise a city block away). Also note that this original correlation of organ and voices fulfilled a different purpose than that which was ascribed to the organ in the Reformed churches: that of introducing hymn tunes to be sung by a congregation. In both cases, the organ fulfilled the role of amplifying the principal sung music, but the liturgical imperatives differed. In the century or so leading to the present, organ music based upon chant sources has fallen into three predominant categories: 1.) works specifically notated or improvised to support the practice of alternatim (including nearly all the early sources from the late Middle Ages through the Renaissance comprising brief interludes taking no longer than the singing of the chant tune); 2.) works intended for liturgical performance in a more modern context (as mass suites for "organ masses," that were Low Masses during which organists played extended movements as Entrances, Offertories, Communions, and Postludes, an idiom that flourished until about 1965 with signal contributions from the French composers like Langlais, Tournemire, and Litaize, and the improvisors led by Cochereau); 3.) larger concert works founded on Gregorian melodies.Each of these essays on chant themes is only an attempt to pictorialize the text, the event, the setting. The performer, and for that matter the listener, is invited to experience the colors of the stained glass, the expert voices of the schola, and the smell of the incense!