Ever since the Baroque Period, almost all music had been written in one of two kinds of scales: major or minor. By the end of the Romantic Period, however, these scale systems were growing tired. The increasingly intense emotions of the time could not be captured by using just the seven availible tones of a given key. Increasingly, composers began to use notes from the chromatic scale to create a greater sense of emotion and tension.
The chromatic scale includes all 12 tones of the octave, whereas the major and minor scales only use seven of the twelve. By using the tones that are not "supposed" to be in a certain key, composers were able to create stronger and more effective dissonances. In turn, the exaggered dissonance created more tension, which gave a greater sense of relief when the music arrived at a moment of release. Alternatively, the moment of release could be delayed using chromatic harmonies, so that the listener is constantly pulled forward, waiting for the resolution. This can be heard in the operas of Richard Wagner, where the melodies seem to drag on forever.
As composers used more and more chromatic tones, the sense of
key was gradually weakened. This is because the old tonal system
was designed to gravitate around its key center. Chromaticism
served to dilute this effect with the added "outside" notes.
Eventually, this lack of tonal focus grew to the point of atonality, where there was no longer any
indentifiable tonal center.