Contrast chord example Play (help·info): C major and E minor contrast through their respective notes C and B (in red and orange), each a half step apart or leading tones. The chords share two notes (in blue) however.
In music, a contrast chord is an auxiliary chord formed from one of the three primary triads and sharing its diatonic function. Leittonwechselklänge (German: "leading-tone contrast chords"), abbreviated Tl in major and tL in minor.
If chords may be formed by raising (major) or lowering (minor) the fifth a whole step [ parallel or relative chords], they may also be formed by lowering (major) or raising (minor) the root a half-step to wechsel, the leading tone or leitton. These chords are Leittonwechselklänge (literally: "leading-tone changing sounds"), sometimes called gegenklang or "contrast chord". 
Major Leittonwechselklänge, formed by lowering the root a half step.
Minor Leittonwechselklänge, formed by raising the root (US)/fifth (German) a half step.
^Gjerdingen, Robert O. (1990). "A Guide to the Terminology of German Harmony", Studies in the Origin of Harmonic Tonality by Dahlhaus, Carl, trans. Gjerdingen (1990). Princeton University Press. ISBN 0-691-09135-8.