the hymn melody is given out with one hand or upon the pedals, while around it is woven a network of freely moving parts. The prelude may be brief, included within the space limits of the original melody, or it may be indefinitely extended by increasing the length of the choral notes and working out interludes between the lines. The one hundred and thirty choral preludes which have come down to us from Bach’s pen are samples of the kind of thing that he was extemporizing Sunday after Sunday. In these pieces the accompaniment is sometimes fashioned on the basis of a definite melodic figure which is carried, with modulations and subtle modifications, all through the stanza, sometimes on figures whose pattern changes with every line; while beneath or within the sounding arabesques are heard the long sonorous notes of the choral, holding the hearer firmly to the ground idea which the player’s art is striving to impress and beautify. This form of music is something very different from the “theme and variations,” which has played so conspicuous a part in the modern instrumental school from Haydn down to the present. In the choral prelude there is no modification of the theme itself; the subject in single notes forms a _cantus firmus_, on the same principle that appears in the mediaeval vocal polyphony, around which the freely invented parts, moving laterally, are entwined. Although these compositions vary greatly in length, a single presentation of the decorated choral tune suffices with Bach except in rare instances, such as the prelude on “O Lamm Gottes unschuldig,” in which the melody is given out three times, with a different scheme of ornament at each repetition.