When Giulio Cesare Monteverdi coined the term seconda prattica in the preface to his brother Claudio'sScherzi musicali of 1607, he not only defended his brother's new style of composition, he also articulated two ideals that would be shared by composers for the next century and a half. If the first compositional "practice"--the practice of the Renaissance--had been to strive for contrapuntal perfection as defined by the balancing of consonance and dissonance, the second "practice," ("baroque," as it came to be known) advocated two more ideals: that the musical motive ought to represent emotional states of being, and that all parts must be subservient to a fundamental bass line.
Doctrine of Affections
The "Doctrine of the Affections" was first promulgated at the end of the Renaissance when a group of Florentine academics attempted to restore what they perceived to be the pure word-to-music relationships advocated by classical Greek philosophers such as Plato. The doctrine revealed itself in the seventeenth and eighteenth centuries as a presupposition that the motivic germ of a composition, its inventio, was more than mere representation, but a tangible embodiment of Affekt: an emotional state of being. It was believed, for example, that a lamento bass was the palpable expression of sadness, while a rapidly rising sequence of thirds was the opposite--euphoria. This "doctrine" is clearly spelled out in numerous theoretical tomes of the eighteenth century, not the least of which are treatises by Bach's own cousin, Walther, and other contemporaries such as Heinichen and Mattheson.
Baroque composers manifested their second ideal in the ubiquitous basso continuo, a fixture that perhaps yet more than Affekt was the essential characteristic of baroque style. The score of a continuo part, known as figured bass, consisted of a bass line accompanied by numbers (figures) representing the remaining pitches of each chord. The bass line was played by a sustaining low-pitch instrument, typically 'cello or bassoon, while the figures were "realized" by a chording instrument such as the harpsichord, organ, or lute. Above this fundamental bass/chord structure the baroque musician played other parts with the expectation that copious ornaments and improvisations might be invoked. The basso continuo idiom is, in many respects, analogous to a contemporary jazz lead sheet.
The figured bass technique was so important that it comprised the essential pedagogical element in the musical education of composers from Schütz to Mozart and it gave rise to a generation of music theorists--Descartes, Rameau, Sorge, Marpurg, Kirnberger, and Tartini--some of whose ideas came themselves to be known by the collective nomology "figured-bass school."
Rameau & Fux
Although he surely fancied himself more as composer than as theorist, Jean-Philippe Rameau's great gift to the world was his concept of chord generation from the fundamental bass and the simple, albeit revolutionary, notion of the invertibility of the triad. His Traité de l'Harmonie not only explained the generation of the seventh chord upon the basis of tertian theory, it was developed upon propositions strongly suggestive of the yet undiscovered harmonic series. It is reported that J. S. Bach was familiar with Rameau's theory of chord inversion, but that he rejected it.
But Johann Sebastian was clearly more sympathetic to the ideas of Johann Joseph Fux, whose Gradus ad Parnassum (1725), a systematic theory of Renaissance species counterpoint in the style of Palestrina was translated from its original Latin to German, in Leipzig, eight years before Bach's death. A document from this period confirms that Johann Sebastian owned a copy (of the Latin version), and Carl Philipp Emanuel wrote that Gradus left its print upon his father's later works.
End of the Baroque
If Claudio Monteverdi defined a second practice, he merely followed the lead of the Florentines who had, by the late-sixteenth century, developed a monadic idiom that had toyed with baroque style. But it was Claudio who nearly single-handedly began the baroque--in music at least. And if Claudio's basso continuo madrigals began it, then the death of Johann Sebastian in 1750 brought it to a close.
Musical giants, no two individuals more unlike each other could be envisioned. Monteverdi, patrician artisan of Mantua's house of Gonzaga and later maestro di cappella at St. Mark's Cathedral in Venice, was a transitional figure, like Beethoven, who stood with one foot in the past and the other treading into the future. By contrast, Bach, riding as it were the last crest of the baroque wave and ever conscious of what had transpired before him, synthesized the best of the baroque and elevated it to a dizzying height of perfection that could not possibly have been sustained after him. Not to oversimplify the comparison between the two, it is probably accurate to characterize the former as a radical and the latter as a conservative; both climbed Parnassus, but by different steps.