The flats

A flat (b) means: one key to the LEFT. The black key directly left of D is D-flat, written Db.

Notice: Db is the SAME key as C#. Every black key has two names — one with a sharp, one with a flat. Which name is used depends on the musical context; you will meet both, so learn both.

The five black keys as flats: Db, Eb, Gb, Ab, Bb.

b = one key to the left · C# and Db are the same key

The same five black keys, named as flats.

Practise this in the trainer — it opens with this lesson the first time the concept comes up.