Music after the Romantic era experienced a great deal of experimentation and change as many of the old methods that had formed the basis of classical music for centuries were challenged. Tonality, for example, was discarded entirely by many composers in the 20th century. Notably, a number of distinct movements in classical music developed as well.
Impressionism is the name given to a movement in painting that emerged primarily in France during the latter part of the 19th Century. It featured visual renderings that were intended more for decoration than as records of the precise appearance of people or objects.
The invention of photography provided a faster, cheaper means of recording appearances. Impressionism, instead, was a visual means of recording supposed "impressions" of the subject matter--emphasizing selected features while minimizing others. In practical terms, impressionistic effects were achieved by (1) reducing the detail in a picture, (2) eliminating subtle color mixtures, and (3) exaggerating proportions and perspective to convey motion or lassitude.
The composer Debussy is said to have resisted the comparisons of his music to the paintings of the Impressionists, but the vague melodic and harmonic structures of works such as "L'après-midi d'une faune", "Syrinx" (for Solo Flute) and "Nuages" seem to be in much the same spirit as the blurred, sensuous visual images of the painters with whom he would have been familiar.
Again in practical terms, Debussy was able to differentiate his compositions from what had come before by deliberately writing music that was contrary to the norms that he and every other young composer were taught at the Conservatoire. He abandoned the major and minor scales for other combinations of pitches. He also abandoned 7-note scales for groupings of five notes (pentatonic scales) and six notes (whole-tone scales).
Debussy frequently based his harmonies on so-called "parallel fifths." These were produced by simultaneous melodies that were always exactly seven semitones apart, and they were strictly forbidden by the common practice taught at the time.
Debussy's experiments with new ways of organizing music would not have been nearly as successful if he had not possessed true genius. Even his most vigorous critics were compelled to acknowledge his substantial talent.
He continued to exploit this style of music from 1891, when he introduced "L'après-midi d'une faun" until his death in 1917. He made such thorough and brilliant use of the "impressionist" techniques that other composers were hard pressed to successfully follow his lead.
Other notable composers who are usually categorized as impressionists include Maurice Ravel and the American composer Charles Tomlinson Griffes. In the generation that followed, Darius Milhaud, Selim Palmgren, and others were obviously influenced by Debussy's music but found their own paths by incorporating American jazz or German "expressionist" elements.
Atonal music is characterized by the absence of a key or tonal center.
Serialism, in its purest form, is a style of music in which each tone in the twelve note chromatic scale is used exactly once in succession until all twelve are used, at which time the cycle may be repeated using the same series of notes or a similar series derived from the original. The twelve note series is sometimes referred to as a "tone row". From this initial row of tones, a matrix can be calculated by inverting the intervals of the row to form a column descending from the first note of the row and then creating new rows by transcribing the intervals of the original row beginning on each of the tones of the inversion. By aligning these rows and columns in a twelve by twelve grid, one derives a total of 48 variations of the row by beginning at any point on the outside of the grid and proceeding vertically or horizontally, as the case may be. A tone row from left to right is referred to as "prime" as it is the initial set of intervals. A column from top to bottom is referred to as an inversion. From right to left is called "retrograde". Consequently, from bottom to top is "retrograde inversion". Any one of these 48 rows may be used but only one row is typically used per part at any given time. However, two parts playing simultaneously - voice and piano, for instance - can use completely separate rows at the same time. At some time after the initial development of serialism composers began using "subsets", or fragments of a row repeated. This allowed freedom to create compositions that were more pleasing to the traditional ear.
This style of composition was initially developed in Austria by Arnold Schoenberg, and was further developed by students of his, including Anton Webern and Alban Berg.