Understanding chord structure is very helpful for improvisation. The following list shows a range of chords with the three letters that make up each chord. All these notes are safe notes and work well if you play them while the specific chord is being played.
The beginning of a chord (typically played by a guitar) is the best time to play any of the notes that make up the chord change. Understanding the timing when chord changes are happening is very important to improvisation and creating a musical sound.
D major chord (D): Made up of notes D, F♯, and A
G major chord (G): Made up of notes G, B, and D
C major chord (C): Made up of notes C, E, and G
A major chord (A): Made up of notes A, C♯, and E
E major chord (E): Made up of notes E, G♯, and B
B major chord (B): Made up of notes B, D♯, and F♯
F major chord (F): Made up of notes F, A, and C
B♭ major chord (B♭): Made up of notes B♭, D, and F
E♭ major chord (E♭): Made up of notes E♭, G, and B♭
A♭ major chord (A♭): Made up of notes A♭, C, and E♭
A minor chord (Am): Made up of notes A, C, and E
B minor chord (Bm): Made up of notes B, E, and G
C minor chord (Cm): Made up of notes C, E♭, and G
D minor chord (Dm): Made up of notes D, F, and A
E minor chord (Em): Made up of notes E, G, and B
F minor chord (Fm): Made up of notes F, A♭, and C
G minor chord (Gm): Made up of notes G, B♭, and D
A major 7 chord (A7): Same as A major with G♯
B major 7 chord (B7): Same as B major with A
C major 7 chord (C7): Same as C major with B
D major 7 chord (D7): Same as D major with C♯
E major 7 chord (E7): Same as E major with D♯
F major 7 chord (F7): Same as F major with E
A minor 7 chord (Am7): Same as A minor with G
B minor 7 chord (Bm7): Same as B minor with A
C minor 7 chord (Cm7): Same as C minor with B♭
D minor 7 chord (Dm7): Same as D minor with C
E minor 7 chord (Em7): Same as E minor with D
F minor 7 chord (Fm7): Same as F minor with E♭