Dominant 7th
Major triad plus a flat 7th — tense and bluesy
The dominant 7th chord is a tetrad that adds the minor 7th to the major triad, giving a tense, bluesy quality that wants to resolve. It is the only tetrad containing a tritone (between its 3rd and ♭7), which gives it strong directional pull. In the major scale it lands on the 5th degree and pulls toward the root — likely a reason the major scale prevails over the other major modes.