The familiar Organ is shown in the Positive and Portable organs (l'orgue positif et portatif), small instruments representing reduced front portions of the Montre, the visible speaking pipes of the mediæval church organ, which was neither more nor less than a large mixture register; that is to say, each key when put down sounded the octave, twelfth, super-octave, and other notes simultaneously with the fundamental note. The movement of the Plain Song, or of any melody with this harmonic structure upon it, was in progressions that no modern musically trained ear could tolerate. Not more than one key of the large church organ could, however, be put down by either hand, as the keys were as broad as the palm, and to press one down required an attack with the player's fist. But the keys of the Portable organ, a processional instrument, were narrow--one hand manipulating the bellows while the player touched the keys with the other. The positive was a chapel or chamber organ, intended to be stationary, and also with narrow keys, admitting of the grasp of an octave. The key-board shown in the Van Eyck St. Cecilia panel of the famous altar-piece at Ghent--the Adoration of the Lamb--has already the complete arrangement of chromatic keys, exactly as in our modern key-board instruments. The date of this panel could not have been later than A.D. 1426. Among the numerous portable organs depicted in paintings of earlier date, the addition of the _ficti_, as the chromatic notes were called, excepting perhaps the B flat, or the B flat and E flat, does not appear. The B flat, however, was not a chromatic, but an essential note in the ecclesiastical scale. Another early instance in a painting by Hans Memling, preserved in the Hospital of St. John at Bruges, is subsequent in date to the Ghent altar-piece, but is still within the fifteenth century. The chromatic keys are here put farther back than in our usual key-boards, as they were also in the fourteenth-century Halberstadt organ. The fourteenth-century portable organ drawn in _Critical and Bibliographical Notes on Early Spanish Music_, by Don Juan F. Riaño (Quaritch, London), 1887, p. 127, shows the B flat only, and figured as an upper key, apparently not raised, but level with the natural keys, which agrees with contemporary representations of the instrument by Fra Angelico. The Portable Organ (Plate XIII.) here drawn is of comparatively late date, these small instruments having remained in use until after the Reformation. The Positive Organ (Plate XI.) is of earlier date, and is so far developed as to have registers and drawstops to govern them; in this example they are an octave apart, but an unimpeachable authority, Praetorius, speaks of a register in a positive organ a fifth from the fundamental one! Our ancestors were evidently not affected by a progression of fifths as we are.