His "Twelve Little Preludes" alone contain the materials for an entire system of music. The "Inventions," too often treated as dry-as-dust studies, are laden with beautiful figures and devices that furnish inspiration for all time. As indicated by their title, which signifies a compound of appropriate expression and just disposition of the members, they were designed to cultivate the elements of musical taste, as well as freedom and equality of the fingers. His "Well Tempered Clavichord" has been called the pianist's Sacred Book. Its Preludes and Fugues illustrate every shade of human feeling, and were especially designed to exemplify the mode of tuning known as equal temperament, introduced into general use by Bach, and still employed by your piano tuner and mine. Forkel, his biographer, has finely said that Bach considered the voices of his fugues a select company of persons conversing together. Each was allowed to speak only when there was something to say bearing on the subject in hand. A highly characteristic motive, or theme, as significant as the noblest "typical phrase," developing into equally characteristic progressions and cadences, is a striking feature of the Bach fugue. His "Suites" exalted forever the familiar dance tunes of the German people. His wonderful "Chromatic Fantasia and Fugue" ushered the recitative into purely instrumental music. As a teacher he was genial, kind, encouraging and in every respect a model. He obliged his pupils to write and understand as well as sound the notes. In his noble modesty he never held himself aloof as superior to others. When pupils were discouraged he reminded them how hard he had always been compelled to work, and assured them that equal industry would lead them to success. He gave the thumb its proper place on the keyboard, and materially improved fingering. Tranquillity and poetic beauty being prime essentials of his playing, he preferred to the more brilliant harpsichord, or spinet, the clavichord, whose thrilling, tremulous tone, owing to its construction, was exceedingly sensitive to the player's touch. The early hammer-clavier, or pianoforte, invented in 1711, by the Italian Cristofori, who derived the hammer idea from the dulcimer, did not attract him because of its extreme crudeness. Nevertheless, it was destined to develop into the musical instrument essential to the perfect interpretation of his clavier music. His son and pupil, Philipp Emanuel Bach (1714-1788), proceeding on the principles established by his illustrious father, prepared the way for the modern pianist. His important theoretical work, "The True Art of Clavier Playing," was pronounced by Haydn the school of schools for all time. It was highly extolled by Mozart, and to it Clementi ascribed his knowledge and skill. In his compositions he was an active agent in the crystallization of the sonata form. From him Haydn gained much that he later transferred to the orchestra.