Defines and illustrates the estalishment and usage of four separate areas of musical signification appearing in orchestral music in the Classical period. The area of most potent use, spanning Classical and Romantic periods, was the 'expressive medium' (a new term). Its oscillating texture and use in flat-key tonalities was a signal denoting the proximity of human affection; it could be applied, however, in all types of music, not simply theatre music, as is shown in examples from Viennese Classical sources.