Again, the 5th is omitted, which helps with fingering on the guitar, but it’s also common practice to stop chords becoming ... root (E) - major 3rd (G#) - dominant 7th (D) - #9th (G).
A dominant 7th is built on the dominant note. It functions as a type of chord V. In this example you will hear both chord I of C Major, and the dominant 7th chord. You can follow along with the ...