Major 6th Chords (6) are common in jazz because they are a good substitute for major chords. The fomula for Maj6 chords are Root-3-5-6. In the key of F, this would be spelled F-A-C-D. In the key of C it would be C-E-G-A. Here are four examples of Maj6 chords
Minor 6th chords (min6) are common replacements for minor chords. The formula for a minor 6th chord is Root-b3-5-6. In the key fo G Major, the spelling would be G-Bb-D-E. In D Major it would be D-F-A-B.
Here are four examples of Cmin6 chords