Diatonic Chords for Keys of:
 C, G, D, A, E, B, and F#

The term diatonic means "of the scale" and it is commonly used to talk about chords that belong to a certain key. They are the foundation of harmony and should be memorized when trying to master a certain key. 

There are 7 chords in each key. The 1, 4, and 5 are major chords. The 2m, 3m, and 6m are minor chords. Lastly, the 7 chord is diminished and isn't used too much in popular music. A generally good rule is to memorize the 1, 4, 5, and 6m of a key first, then the 2m and 3m. Learning this information alone doesn't do much. It's only paired with consistent application and practice with real songs that it's memorized and useful. 

Circle of 5ths (1, 4, 5, and 6m are bolded)

Key of C- C, Dm, Em, F, G, Am, Bº

Key of G- G, Am, Bm, C, D, Em, F#º

Key of D- D, Em, F#m, G, A, Bm, C#º

Key of A- A, Bm, C#m, D, E, F#m, G#º

Key of E- E, F#m, G#m, A, B, C#m, D#º

Key of B- B, C#m, D#m, E, F#, G#m, A#º

Key of F#- F#, G#m, A#m, B, C#, D#m, E#º

It's always good to remember that these chords are built and named from their key's major scale. The more you practice focusing on one key at a time, the more the notes in the scale and the diatonic chords of the key will stick and be easy to recall and use. As they are used and continually practiced, these chords are able to be immediately recalled by a professional musician.