F♯dim7 (also written G♭dim7) is a diminished seventh chord made of 4 notes: F♯, A, C, E♭. It sounds spooky and suspenseful, and you hear it in passing chords, old film scores and dramatic moments; any note of it can be the root.
The library above opens on F♯dim7 with 13 shapes up and down the neck. Tap a shape to hear it, or play it one string at a time.