The diminished triad is another commonly used chord. It is similar in structure to the minor triad, but the fifth scale degree is lowered a half step. Follow these steps to determine the pitches of a diminished triad.
1. Write out the scale degrees under the staff. Just like the major and minor triads, only the numbers 1 through 5 are needed. We will continue with the key of B flat, and create a B flat diminished triad.
2. Write the pattern of whole steps and half steps as they occur between each scale degree (W H W H).
3. Write in the first five pitches of the diatonic scale. Do not skip any pitches or repeat any pitches.
Scale degree 1 to 2 is a whole step. A whole step above B flat is C, so no accidental is needed.
Scale degrees 2 to 3 is a half step. A half step above C is D flat, so we need to add that accidental.
Scale degrees 3 to 4 is a whole step. A whole step above D flat is E flat, so we must write a flat in front of the E.
Scale degrees 4 to 5 is a half step. A half step above E flat is F flat. You might be tempted to write the fifth scale degree as an E, but the correct diatonic spelling is F flat.
The triad consist of scale degrees 1, 3, and 5 played at the same time. Therefore, the notes of a B flat diminished triad are B flat, D flat, and F flat. When these notes are played one at a time, they are called a B flat diminished arpeggio.