can also be played as a major 6th chord). The ii7–V7–Imaj7 progression provides smooth voice leading between the thirds and sevenths of these chords; the third of one chord becomes the seventh of the next chord, and the seventh of one chord moves down a half-step to become the third of the next chord. For example, in the key of C, the standard jazz ii–V–I progression is Dm7–G7–Cmaj7, and the thirds and sevenths of these chords are F–C, B–F, E–B; inverted for smoother voice leading, these become F–C, F–B, E–B. The ii is sometimes replaced by the II7, giving it a more dissonant, bluesy feel; this is especially common in turnarounds. Additionally, the ii can be treated like a temporary minor tonic, and preceded by its own "ii–V", extending the basic progression to a iii–VI–ii–V–I; again, this is quite common in turnarounds (with the iii–VI replacing the I in the second-to-last bar; in the example above, the last two bars would change from F7 | Gm–C7 to Am–D7 | Gm–C7). The ii–V7–I can be further modified by applying a tritone substitution to the V7 chord, replacing it with the II7 chord. This is possible because the II7 has the same third and seventh as the V7, but inverted; for example, the third and seventh of G7 are B and F, while the third and seventh of D7 are F and C, which is enharmonic to B. Performing this substitution (in this case, changing Dm7–G7–Cmaj7 to Dm7–D7–Cmaj7) creates smooth chromatic movement in the chord roots—the root of the ii (D) moves down a half-step to become the root of the II7 (D), which moves down another half-step to become the root of the I (C). The tritone substitution, the substitution of II7 for V7, and the III–VI–II–V extension can be combined in different permutations to produce many different variations on the same basic progression—e.g. iii7–III7–iim7–II7–Imaj7–III7–III7–II7–II7–I7, etc. Classical A ii–V–I progression is part of the vi–ii–V–I progression of root movement by descending fifths, which establishes tonality and also strengthens the key through the contrast of minor and major. In minor, a seventh chord built on the supertonic yields a half-diminished seventh chord, which is a very strong predominant chord. Due to what is considered the harsh nature of root position diminished chords, the ii chord most often appears in first inversion. The ii chord appears in the natural minor scale and may