Diminished triads in root position consist of an interval of a third from the root note to the "third," or middle note, and another interval of a third from the middle note ("third") to the top note, called the "fifth."

The other interval at play in a triad is the distance from the "root" to the "fifth," which is an interval of a fifth.

An intervals of a third can be qualified as either  major or minor.

A major third is four half steps, and a minor third is three half steps.

An interval of a "perfect" fifth is seven half steps, but a "diminished" fifth is only six half steps.

A diminished triad in root position (the root being the lowest note) is made up of an interval of aminor third from the "root" to the "third," and another interval of a minor third from the "third" to the "fifth." This means the distance is from the root to the fifth is one half step short of a "perfect" fifth, and so it is "diminished."

In the chart below the intervals are represented:

Major third = M3
Minor third = m3
Diminished fifth = d5