to be viewed more in terms of its themes or groups of themes, rather than the sharp differentiation of tonal areas based on cadences. In the Classical period, establishing the expectation of a particular cadence and then delaying or avoiding it was a common way of creating tension. In the 19th century, with its dramatically expanded harmonic vocabulary, avoidance of a cadence did not have the same degree of unexpectedness. Instead, more distant key regions were established by a variety of other means, including use of increasingly dissonant chords, pedal points, texture, and alteration of the main theme itself. In the Classical period, the contrast between theme groups, while useful, was not required. The first theme group tended to outline the tonic chord, and the second theme tended to be more cantabile in character. But this was far from universal – as Haydn's monothematic expositions and Beethoven's early rhythmic themes show. Because the power of harmonic opposition, both between tonic and dominant and between major and minor, had less force in the Romantic vocabulary, stereotypes of the character of themes became stronger. Nineteenth-century theorists described the sonata principle as one of opposition between two groups of themes. Thus, it was thought by Marx that the first theme should be "masculine" – strident, rhythmic, and implying a dissonance – and the second theme group should be drawn more from vocal melody making it "feminine". It is this contrast between "rhythmic" and "singing" that Wagner, in his very influential work On Conducting, argued was at the very core of tension in music. That texture, as Wagner argued, is the most important contrast and that tempo should be used to emphasize this contrast. Thus, fast sections were conducted faster and slow sections were conducted slower. By requiring that harmony move with the themes, 19th-century sonata form imposed a kind of discipline on composers and also allowed audiences to comprehend the music by following the appearance of recognizable melodies. However, the sonata form, as an inherited formal mold, also created a tension for Romantic composers between the desire to combine poetical expression and academic rigor. Later Romantic commentators and theorists detected a "sonata idea" of increasing formalization. They drew a progression of works from Haydn, through Mozart and Beethoven, whereby more and more movements in a multi-movement work were felt to be in sonata form. These theorists present the theory that originally only first