All we need to do to create a diminished triad is drop the G by a semitone, to a Gb (aka F#). The effect that this very basic operation has on the sound of your triad is instant. You may notice ...

A diminished triad is built on the 7th note of the scale ... A perfect cadence in the key of C major ends by moving from V (G major) to I (C major) Listen to this example of a perfect cadence ...