The modulation chord
The fully diminished chord is 6 chord, for example C6, with the 3rd and 5th lowered. Or a m6, for example Cm6, with the 5th lowered. However, it’s name comes from traditional theory, where a standard 7 chord, often called a dominant 7 chord, lowers every note but the root. There are only four of them, and each one easily moves to four different keys. For this reason it is a modulation chord.