Common practice period
In the history of European art music, the common practice period is the era of the tonal system. Though it has no exact dates, most features of the common-practice period persisted from the mid- to late baroque period, through the Classical, Romantic and Impressionist periods, from around 1650 to 1900. The period saw considerable stylistic evolution, with some patterns and conventions flourishing and then declining, for example the sonata form. Thus, the dates 1650–1900 are necessarily nebulous and arbitrary borders that depend on context. The most important unifying feature throughout the period is a harmonic language  to which modern music theorists can apply Roman numeral chord analysis. Technical features. Harmony. The harmonic language of this period is known as "common-practice tonality", or sometimes the "tonal system" (though whether tonality implies common-practice idioms is a question of debate). Common-practice tonality represents a union between harmonic function and counterpoint. In other words, individual melodic lines, when taken together, express harmonic unity and goal-oriented progression. In tonal music, each tone in the diatonic scale functions according to its relationship to the tonic (the fundamental pitch of the scale). While diatonicism forms the basis for the  tonal system, the system can withstand considerable chromatic alteration without losing its tonal identity. Throughout the common-practice period, certain harmonic patterns span styles, composers, regions, and epochs. Johann Sebastian Bach and Richard Strauss, for instance, may both write passages that can be analysed according to the progression I-ii-V-I, despite vast differences in style and context. Such harmonic conventions can be distilled into the familiar chord progressions with which musicians analyse and compose tonal music. Various popular idioms of the twentieth century break down the standardized chord progressions of the common-practice period. While these later styles  incorporate many elements of the tonal vocabulary (such as major and minor chords), the function of these elements is not necessarily rooted in classical models of counterpoint and harmonic function. For example, in common-practice harmony, a major triad built on the fifth degree of the scale (V) is unlikely to progress directly to a root position triad built on the fourth degree of the scale (IV), but the reverse of this progression (IV–V) is quite common. By contrast, the V–IV progression is readily acceptable by many other standards; for example, this transition is essential to the "shuffle