One essential element of musical expression is the living sound, capable of holding the constant attention of the audience. This lesson traces that ubiquitous concept from Caccini’s “swelling and abating of the voice” (1602) to the violin études of Mazas (1843). In the Baroque sound world, free from the all-pervasive vibrato of modern times, it was the responsibility of the bow to provide this ‘inner life of sound.’ Based mainly on the writings of Tartini, Geminiani, and Leopold Mozart, all of whom are quoted, this lesson contains five exercises for perfecting the expressive device that guaranteed this living sound, the “Messa di voce.” The many different aspects of its technique, gleaned from the sources, are isolated and explained in detail, from simple pressure with the forefinger to the addition of vibrato: two composers who indicated this device in their compositions, Veracini and Piani, are quoted and illustrated.