Guitar chords by key

The seven chords of C major are C, Dm, Em, F, G, Am, Bdim. Each one has a job in the key, and each tends towards particular chords next.

• C, the tonic of C major. Usually goes to F or G: I moves to IV, which can return directly to I or continue to V. The chord a phrase begins and ends on, and the one everything else is measured against. Played plain it is the version that leaves the colouring to the melody and the bass, which is why a folk or rock arrangement of a tune uses the triad where a jazz chart writes Imaj7 or I6/9.
• Dm, the supertonic of C major. Usually goes to G: roots falling a fifth, the strongest root motion the key has. Used immediately in front of the dominant and hardly anywhere else: ii–V–I with plain triads is the cadence of a hymn or a classical phrase, where a jazz chart would write ii7–V7–Imaj7 and mean the same three chords.
• Em, the mediant of C major. Usually goes to Am: roots falling a fifth, and vi is where iii most often lands. It contains the key's leading tone, which sits a half step below the tonic and commonly rises to it. The least-used diatonic triad, iii most often passes between I and IV or supports a melody note shared with I. Raising its 3rd makes III, the secondary dominant of vi, which gives it a clear resolution target.
• F, the subdominant of C major. Usually goes to G: subdominant into dominant: the standard approach to a cadence. Used two ways, and both are everywhere: as the departure before the dominant (IV–V–I), and as a complete phrase on its own with the tonic (I–IV–I, the plagal "amen"), which needs no dominant and so ends without tension.
• G, the dominant of C major. Usually goes to C: the leading tone rises to the tonic while V7’s flat 7th falls to the 3rd of I. It contains the key's leading tone, which sits a half step below the tonic and commonly rises to it. Nearly always followed by the tonic or by vi; anything else is a deliberate refusal. Without the 7th the pull is a suggestion rather than an insistence, which is exactly why hymns, folk tunes and most rock use the plain triad here and jazz almost never does.
• Am, the submediant of C major. Usually goes to Dm or G: roots falling a fifth, straight into the standard approach to V. Used as the tonic's substitute and as the key's minor colour: the chord a deceptive cadence lands on, and the chord a whole section of a major-key song can be built on without any modulation happening at all.
• Bdim, the leading-tone chord of C major. Usually goes to C: vii° contains V7’s leading tone and tritone, so it commonly resolves to I. The root and ♭5 form a tritone. Because those notes usually resolve by step into the next chord, this chord is most often used as a passing or predominant chord. It contains B and F, the same tritone found in V7. B can rise to the tonic while F falls to the 3rd of the tonic chord. It contains the key's leading tone, which sits a half step below the tonic and commonly rises to it. Rarely played in root position at all. Its normal appearance is with the 3rd in the bass, as a passing chord between the tonic and the tonic in first inversion -- a dominant that is never allowed to become a cadence.