The major third stacked on top of a minor third forms a major seventh chord. In this context, if you start with a root note, add a major third above it, and then add a minor third above that major third, you create a seven-part structure typical in jazz and classical harmony.
For example, if you take the note C as the root:
- The major third above C is E.
- The minor third above E is G.
If you consider the next third which forms the seventh, you would add B (the major third above G). Thus, the notes C, E, G, and B together form a C major seventh chord (Cmaj7).