The first chord in Wagner's Tristan und Isolde, f-b-d#′-g#′. The name has been used for any four-note chord constructed with the same intervals (sometimes called a ‘half-diminished 7th chord’ in American usage); its harmonic significance in Wagner's opera has led it to be viewed as the basis of a ‘crisis’ in Romantic harmony.

The Concise Grove Dictionary of Music. Copyright © 1994 by Oxford University Press, Inc.. All rights reserved.