10.10Why diminished seventh notation looks strange

You may wonder:

Why not simply call the last note A?

Because chord spelling represents interval structure.

From C:

C → E♭ = minor 3rd

C → G♭ = diminished 5th

C → B𝄫 = diminished 7th

Even though B𝄫 and A are the same piano key in equal temperament, theoretically they have different functions.

At the piano, don't let the notation scare you.