Chromaticism, according to the Harvard Dictionary of Music, "serves to heighten the emotional tension of music." It consists of the 12-tone scale, the basis of atonal music. But chromatic harmony is not necessarily pure atonality, as one heard last night in a concert by the Twentieth Century Consort in league with the American String Quartet at the Library of Congress.

The event was a fascinating exploration of the possibilities of chromaticism, going back chronologically to Johann Sebastian Bach, with an arrangement from the 1930s by the atonalist Anton Webern of that craggy, harmonically ambivalent product of Bach's old age, the six-voice fugue from "A Musical Offering." In this version, with the emphasis on the winds and brass, it becomes almost a fugal fanfare -- briskly played last night.

The newly proclaimed musical trend of Neo-Romanticism teeters on the narrow divide between chromatic and diatonic harmony. The two works on the first half of last night's program demonstrated how narrow is the narrowness. One was that glorious little serenade, the "Siegfried Idyll" by Wagner, the man who opened the door to atonality in "Tristan." The other was Berg's Lyric Suite, a committed, eloquent exercise in atonality for string quartet that takes a symbolic bow to "Tristan's" moment of atonality in a last-movement quote. Both are passionate works, and they were passionately played last night.

There was also Webern's wispy, shadowy little four-movement work of minimalism, "Vier Stu cke," memorably played by violinist Mitchell Stern and pianist John Mugge. The full ensemble (about 20 players) played delectably Scho nberg's "Kammersymphonie," which is something like Mozart's "Eine kleine nachtmusik" harmonically updated a century and a quarter.