E9 is a dominant ninth chord made of 5 notes: E, G♯, B, D, F♯. It sounds funky and full, a seventh chord with more colour, and you hear it in funk, soul and blues, where it often replaces a plain 7th chord. It is the V chord of A major and turns up in 2 other keys.
The library above opens on E9 with 14 shapes up and down the neck, starting with hand-checked ones. Tap a shape to hear it, or play it one string at a time.