Guitar Chords in the Key of C Major

The chords in the key of C Major

C Major Chord C
D minor Chord Dm
E minor Chord Em
F Major Chord F
G Major Chord G
A Minor Chord Am
B Diminished Chord Bdim

C Major - D minor - E minor - F Major - G Major - A minor - B diminished (B°).

      C     Dm    Em    F     G     Am    Bdim  
      I     ii    iii   IV    V     vi    vii   

The chord formula for any Major key is
Major - minor - minor - Major - Major - minor - diminished

A common way to number these chords is by Roman numerals
I - ii - iii - IV - V - vi - vii

(Major chords are usually capitalized, minor and diminished chords are lower case)