20th century composers experiemented with music by:

Early twentieth-century composers wanted their music to sound different from the music of the previous generation but they found that Romantic composers had pushed the boundaries of tonal harmony as far as possible.
One of the basic principles of tonal harmony is that dissonant sounds create tension and have to be resolved to consonant sounds.
Romantic composers made their music sound different from the music of the Classical Period by using new dissonant chords, as well as by prolonging the dissonant sound before eventually resolving them to consonant sounds.

If your (k12) then it's all of them.i learned it the hard