The late 19th and early 20th centuries witnessed major changes, especially in the Euro-American world.
These in turn influenced social, political, philosophical, and artistic changes.
Technology
Impressionism is a 19th-century art movement characterized by relatively small, thin, yet visible brush strokes, open composition, emphasis on accurate depiction of light in its changing qualities (often accentuating the effects of the passage of time), ordinary subject matter, inclusion of movement as a crucial element of human perception and experience, and unusual visual angles. Impressionism originated with a group of Paris-based artists whose independent exhibitions brought them to prominence during the 1870s and 1880s.
The Impressionists faced harsh opposition from the conventional art community in France. The name of the style derives from the title of a Claude Monet work, Impression, soleil levant (Impression, Sunrise), which provoked the critic Louis Leroy to coin the term in a satirical review published in the Parisian newspaper Le Charivari.
The development of Impressionism in the visual arts was soon followed by analogous styles in other media that became known as impressionist music and impressionist literature.
Musical Impressionism is the name given to a movement in European classical music that arose in the late 19th century and continued into the middle of the 20th century. Originating in France, musical Impressionism is characterized by suggestion and atmosphere, and eschews the emotional excesses of the Romantic era. Impressionist composers favoured short forms such as the nocturne, arabesque, and prelude, and often explored uncommon scales such as the whole tone scale. Perhaps the most notable innovations of Impressionist composers were the introduction of major 7th chords and the extension of chord structures in 3rds to five- and six-part harmonies.
The influence of visual Impressionism on its musical counterpart is debatable. Claude Debussy and Maurice Ravel are generally considered the greatest Impressionist composers, but Debussy disavowed the term, calling it the invention of critics. (Wikipedia)
(Achille) Claude Debussy 22 August 1862 – 25 March 1918) was a French composer. He is sometimes seen as the first Impressionist composer, although he vigorously rejected the term. He was among the most influential composers of the late 19th and early 20th centuries.
With early influences including Russian and far-eastern music, Debussy developed his own style of harmony and orchestral colouring, derided – and unsuccessfully resisted – by much of the musical establishment of the day. His works have strongly influenced a wide range of composers including Béla Bartók, Olivier Messiaen, George Benjamin, and the jazz pianist and composer Bill Evans. Debussy died from cancer at his home in Paris at the age of 55 after a composing career of a little more than 30 years.
In 1889, at the Paris Exposition Universelle, Debussy first heard Javanese gamelan music. The gamelan scales, melodies, rhythms, and ensemble textures appealed to him, and echoes of them are heard in "Pagodes" in his piano suite Estampes.. (Wikipedia).
Here is an example of Javanese gamelan, consisting primarily of bronze gongs and chimes
https://www.youtube.com/watch?v=p3HwqqiVxbE
Here is an example of Chinese classical music
https://www.youtube.com/watch?v=3cXIvBSRmzI&t=50s
In his break with tonality (conventional use of scales and harmonies), he often utilized a system of establishing an exotic sounding scale pattern, moving to another one, then returning to the original sonority--roughly analogous to the path followed by sonata allegro form. In his piano piece, "Voiles" (Veils) he establishes a whole-tone scale sound, vaguely evocative of Javanese gamelan scales, then shifts to a Chinese-sounding pentatonic (5-note) scale, then back again. https://www.youtube.com/watch?v=VrVyQhUM5C4
Joseph Maurice Ravel 7 March 1875 – 28 December 1937) was a French composer, pianist and conductor. He is often associated with impressionism along with his elder contemporary Claude Debussy, although both composers rejected the term. In the 1920s and 1930s Ravel was internationally regarded as France's greatest living composer.
Boléro is "Ravel's most straightforward composition in any medium".[6] The music is in C major, 3
4 time, beginning pianissimo and rising in a continuous crescendo to fortissimo possibile (as loud as possible). It is built over an unchanging ostinato rhythm played on one or more snare drums that remains constant throughout the piece:
On top of this rhythm two melodies are heard, each of 18 bars' duration, and each played twice alternately. The first melody is diatonic, the second melody introduces more jazz-influenced elements, with syncopation and flattened notes (technically it is mostly in the Phrygian mode). The first melody descends through one octave, the second melody descends through two octaves. The bass line and accompaniment are initially played on pizzicato strings, mainly using rudimentary tonic and dominant notes. Tension is provided by the contrast between the steady percussive rhythm, and the "expressive vocal melody trying to break free".[17] Interest is maintained by constant reorchestration of the theme, leading to a variety of timbres, and by a steady crescendo. Both themes are repeated a total of eight times. At the climax, the first theme is repeated a ninth time, then the second theme takes over and breaks briefly into a new tune in E major before finally returning to the tonic key of C major.
https://www.youtube.com/watch?v=GJVWEstu_lM
Here is a reconstruction of the ballet setting. Ravel meant it to represent impersonal factory automation.
https://www.youtube.com/watch?v=UYE125CEnMQ
"Boléro was given its first performance at the Paris Opéra on November 20, 1928. The premiere was acclaimed by a shouting, stamping, cheering audience in the midst of which a woman was heard screaming: “Au fou, au fou!” (“The madman! The madman!”). When Ravel was told of this, he reportedly replied: “That lady… she understood.”
(https://www.classicfm.com/composers/ravel/guides/story-maurice-ravels-bolero/, accessed 6 May, 2020)
Expressionism is a modernist movement, initially in poetry and painting, originating in Germany at the beginning of the 20th century. Its typical trait is to present the world solely from a subjective perspective, distorting it radically for emotional effect in order to evoke moods or ideas. Expressionist artists have sought to express the meaning of emotional experience rather than physical reality.
Expressionism developed as an avant-garde style before the First World War. It remained popular during the Weimar Republic, particularly in Berlin. The style extended to a wide range of the arts, including expressionist architecture, painting, literature, theatre, dance, film and music.
The term is sometimes suggestive of angst. (Wikipedia)
One well known expressionist painting is "The Scream" by the Norwegian painter Edvard Munch.
It is said that Munch suffered from, or was familiar with, the panic attacks associated with agoraphobia (fear of open spaces). The new science of psychoanalysis, with the Freudian model of the id, superego, and ego, very possibly influenced this movement, as did experimentation, especially in France with drugs, such as the psycho-active drink absinthe.
The term expressionism "was probably first applied to music in 1918, especially to Schoenberg", because like the painter Kandinsky he avoided "traditional forms of beauty" to convey powerful feelings in his music. Arnold Schoenberg, Anton Webern and Alban Berg, the members of the Second Viennese School, are important Expressionists (Schoenberg was also an expressionist painter). ....
Theodor Adorno describes expressionism as concerned with the unconscious, and states that "the depiction of fear lies at the centre" of expressionist music, with dissonance predominating, so that the "harmonious, affirmative element of art is banished" (Adorno 2009, 275–76). Erwartung and Die Glückliche Hand, by Schoenberg, and Wozzeck, an opera by Alban Berg (based on the play Woyzeck by Georg Büchner), are examples of Expressionist works. If one were to draw an analogy from paintings, one may describe the expressionist painting technique as the distortion of reality (mostly colors and shapes) to create a nightmarish effect for the particular painting as a whole. Expressionist music roughly does the same thing, where the dramatically increased dissonance creates, aurally, a nightmarish atmosphere. (Wikipedia)
Arnold Schoenberg (1874 – 1951)
The use of atonality, music with no sense of harmony or center is meant to create a sense of uneasiness. Schoenberg and his colleagues also used a voice style known as Sprechstimme ("Speech Voice"), which sounds like pitch chanting, as opposed to actual singing. The following example is his 1912 work for reciter and chamber ensemble, "Pierrot Lunaire" (Moonstruck Pierrot).
https://www.youtube.com/watch?v=bd2cBUJmDr8&t=158s
A more serious example is his 1947 cantata for reciter, chorus, and orchestra "A Survivor from Warsaw." The Wikipedia article with the full text and YouTube link are given below.
https://en.wikipedia.org/wiki/A_Survivor_from_Warsaw
https://www.youtube.com/watch?v=rGWai0SEpUQ
Polytonalityi s the musical use of more than one key simultaneously. It was popularized in the 20th Century by the Russian composer Igor Stravinsky (1882 – 1971).
Let's look at two examples. In the 1918 "L'histoire du soldat" (The Soldier's Tale), a suite or group of pieces, we begin with a march starting at time mark 2:00 going through 3:46. In the string bass, we have a pattern often referred to as an "oompah bass" (because tubas make that sort of sound). However, the melody lines are in totally different keys which clash. Nevertheless, the constant bass keeps us focused, so that we are alternately grounded and unsettled--as the composer intended.
https://www.youtube.com/watch?v=d77jzEVcohA&t=212s
Stravinsky's most famous and groundbreaking work was his music for the 1913 ballet, "Sacre du Printemps" (Rite of Spring), with choreography by Vaslav Nijinski and costumes and sets by Nicholas Roehrich.
https://www.youtube.com/watch?v=YOZmlYgYzG4&t=718s
Stravinsky combines polytonality, multiple short repeating patterns (ostinato), and irregular rhythms. This was combined with dancers made up and moving like marionettes on strings. The beginning, like an overture, has many short repeating motives, representing the many birds waking up with the coming of spring.
Dance music was important in African-American life, even from slavery times. "Negro fiddlers" were often called up to play for social events of slave owners. This music mixed together European dance styles with African vocal style and instruments like the banjo, and instruments probably descended from the Senegambian ngoni. https://www.youtube.com/watch?v=qWufK2qRKrQ
This instrument evolved in the Afro-American context to become the banjo. Here is an example of the original, fretless minstrel banjo, played by the award-winning, brilliant founder of the Carolina Chocolate Drops, Rhiannon Giddens. https://www.youtube.com/watch?v=uTNq9s1NmEI
Marching Bands as Public Entertainment
Although not specifically an African-American genre, marches had an important influence on other genres. With the advent of the Industrial Revolution and the development of modern wind instruments in the 19th century, marching bands became popular, not only for parades but for open-air concerts in warm weather. Many towns throughout Western Europe and North America had band shells or pavilions in public parks where concert bands gave performances. One of the most prolific of the American composers in this genre was John Phillip Sousa (1854 –1932), known as The March King. Among his greatest hits are "The Stars and Stripes forever" (The national march of the USA) and "The Liberty Bell."
https://www.youtube.com/watch?v=a-7XWhyvIpE
https://www.youtube.com/watch?v=x7FD9PNpfpo
Ragtime
African Americans were not welcomed into the white-dominated classical music scene in the USA, even though there were many classically trained African-American musicians and composers. Many of these musicians were forced make a living in what we might call the demi-monde (half world) of bars, saloons, dance halls, and bordellos. A piano style evolved, with influences from vigorous dances like the cake walk, march rhythms and sectional structure, and the off-the-beat syncopation coming from Afro-Caribean rhythms coming via New Orleans. With a steady bouncing pattern in the left-hand bass line going constantly, the melody in the right hand is syncopated in a style called "ragging." The style called ragtime, is best known through the works of Scott Joplin (1868-1917)
His best known work is "The Maple Leaf Rag" (1899). Here is an actual mechanical recording of Joplin playing recorded on a player piano roll. https://www.youtube.com/watch?v=jIjpB49bacM. Another favorite, "The Entertainer," was popularized in the 1970s in the Hollywood movie "The Sting." This recording is also played by Joplin on a piano roll https://www.youtube.com/watch?v=2CWVHwlkIhI
Blues
One of the distinctive features of African and Afro-diasporic musics is the format known as call-and-response, in which a leader sings a phrase, which then repeated by a group (cf. Salsa and Meringue). Early records of this in North America are field hollers and work songs. Here is "Long John" lead by a lead singer nicknamed Lightning . He sang with a group of Afro-American convicts at Darrington State Prison Farm, Sandy Point, Texas, 1934. It was recorded by John and Alan Lomax.
https://www.youtube.com/watch?v=4G5KtQynWvc
This format often was combined with pentatonic (5-note scale) melodies derived from West African styles like those of Mali.
Traditional Malian Bambara : https://www.youtube.com/watch?v=gbxcAi6T1yE
Modern Malian Bambara: https://www.youtube.com/watch?v=OgNFwXC4I6M
What evolved in the late 19th to early 20th Century was a format that goes as follows:
First statement of the theme: A
Second statement of the them with contrasting harmony: A1
Contrasting or capping theme B
Each of these sections is generally 4 measures or bars long, so that the form came to be known as the 12-bar blues form.
A classic example is the 1936 recording of Robert Johnson's "Teraplane Blues" The Teraplane was a fancy car manufactured by the Hudson Motor Company. In the song, Johnson compares his girlfriend's body to a car--with hugely sexual allusions.
https://www.youtube.com/results?search_query=teraplane+blues
"Terraplane Blues"
And I feel so lonesome, you hear me when I moan
When I feel so lonesome, you hear me when I moan
Who been drivin' my Terraplane, for you since I been gone.
I'd said I flash your lights, mama, you horn won't even blow
(Somebody's been runnin' my batteries down on this machine)
I even flash my lights, mama, this horn won't even blow
Got a short in this connection, hoo well, babe, it's way down below
I'm gion' heist your hood, mama, I'm bound to check your oil
I'm goin' heist your hood, mama, mmm, I'm bound to check your oil
I got a woman that I'm lovin', way down in Arkansas
Now, you know the coils ain't even buzzin', little generator won't get the spark
Motor's in a bad condition, you gotta have these batteries charged
But I'm cryin', pleease, pleease don't do me wrong.
Who been drivin' my Terraplane now for, you since I been gone.
Mr. highway man, please don't block the road
Puh hee hee, please don't block the road
'Cause she's reachin' a cold one hundred and I'm booked and I got to go
Yoo... you hear me weep and moan
Who been drivin' my Terraplane now for, you since I been gone
I'm gon' get down in this connection, keep on tanglin' with your wires
I'm gon' get down in this connection, oh well, keep on tanglin' with these wires
And when I mash down on your little starter, then your spark plug will give me fire
As blues moved up the Mississippi River, it became more up-tempo, evolving into Rhythm and Blues. Acoustic versions were exemplified by musicians like John Lee Hooker https://www.youtube.com/watch?v=rS3D7n5EMPY
Starting in the 1940s blues mixed with gospel, another notable African-American genre, adding electric guitar, and rhythm section with drums, bass, and piano and/or guitar. A recently rediscovered giant in this trend is Sister Rosetta Tharpe, known as the Mother of Rock and Roll.
https://www.youtube.com/watch?v=Y9a49oFalZE.
People influenced by her include Chuck Berry https://www.youtube.com/watch?v=6ROwVrF0Ceg
and Elvis Presley https://www.youtube.com/watch?v=lzQ8GDBA8Is
Musical Theater and Popular Songs
As noted in the discussion of music in the 19th Century, popular theater like vaudville and minstrel shows did much to spread popular music, later sold as sheet music for home consumption. Full-blown musical shows alternating dialog and songs became a set format starting in the late 19th Century. Many songwriters came out of minority communities: Irish, Ashkenazic (Eastern European Jewish), and African American.
Tin Pan Alley was the collection of New York City music publishers and songwriters who dominated the popular music of the United States in the late 19th century and early 20th century. The name originally referred to a specific place: West 28th Street between Fifth and Sixth Avenues in the Flower District of Manhattan; a plaque (see below) on the sidewalk on 28th Street between Broadway and Sixth commemorates it. In 2019 the New York City Landmarks Preservation Commission took up the question of preserving five buildings on the north side of the street as a Tin Pan Alley Historic District.
The start of Tin Pan Alley is usually dated to about 1885, when a number of music publishers set up shop in the same district of Manhattan. The end of Tin Pan Alley is less clear cut. Some date it to the start of the Great Depression in the 1930s when the phonograph, radio, and motion pictures supplanted sheet music as the driving force of American popular music, while others consider Tin Pan Alley to have continued into the 1950s when earlier styles of music were upstaged by the rise of rock & roll, which was centered on the Brill Building. (Wikipedia).
The standard song form coming out of musical theater and Tin Pan Alley is known as the 32-bar song form
Intro--Verse--Verse-Bridge-Verse (coda)
A familiar example (except for the rarely performed intro) is "Somewhere Over the Rainbow" from the musical and movie "The Wizard of Oz." https://www.youtube.com/watch?v=hc2rTt74Qys.
Like as arias could be either part of operas or stand-alone pieces, popular songs could either be embedded in Broadway-style or stand-along pop songs. It should be noted that the introduction could be optional if performed in other than a show. Songs of this type were written for performance by big bands in the 1930s through early 50s in a style known as swing jazz (and the accompanying dance style). The band leaders (often the main song writers) were often from minority communities: Irish, African-American, Ashkenazic Jewish.
One of the most famous band leaders was:
William Kennedy (Duke) Ellington (1899-1974) 
One of his best known songs, which was initially a purely instrumental piece with only a very short introduction, was "Take the A Train." The lyrics came later. https://www.youtube.com/watch?v=cb2w2m1JmCY .
Jazz musicians based improvisation on the chord patters of each song, an extension of improvisation on 12-bar blues pattern, but with more variety. Afro-American vocalists developed a style known as skat singing which uses rapid articulation of notes using nonsense syllables. One of the great exponents of this and other jazz vocal styles was Ella Fitzgerald (1917-1996). Here is here improvisations on the Ellington tune https://www.youtube.com/watch?v=dQnNnPLC_b4.
Charlie Parker (1920-1955) and the rise of Bebop
https://www.youtube.com/watch?v=Z2tvlp7RnlM
Combination of Symphonic forms, opera, Broadway, and JazzL
Leonard Bernstein (August 25, 1918 – October 14, 1990) was an American composer, conductor, pianist, music educator, author, and lifelong humanitarian. He was one of the most significant American cultural personalities of the 20th century.
His 1957 musical West Side Story combines lyrics by Steven Sondheim, and choreography by Jerome Robbins. It is a version of Romeo and Juliet set in 1950 gang warfare. Let's look at Bernstein's hybrid approach. The song "Tonight" is a variant on a 32-bar song form (starting at time mark 1:21) https://www.youtube.com/watch?v=m7xTvb-FAhQ&t=237s
This then gets woven into the preparation for the big showdown between the Sharks and the Jets. We have polyphonic contrasts, similar to the scene "Cosa Sento" from Mozart's The Marriage of Figaro.
https://www.youtube.com/watch?v=MyUV3hIL-G0
