On a piano, the simplest key is the C major key. The C major scale starts from the middle C key, then the seven white keys to the right are pressed in order. The notes are named as follows: C-D-E-F-G-A-B-C. This is known as the diatonic order of the piano. There are various other keys, but every one involves the black keys as sharp and flat notes are used.
Why does the diatonic order – the simplest scale – start at C instead of A?
When the modern system of written music came to be, the lowest available note was named “A” for simplicity, then each note above it was named alphabetically. However, at the time the notes were not matched to any specific scales. Furthermore, they started by only using seven letters, but later agreed on a 12-note octave. To make room for the extra five notes, they invented accidentals – the flats (b) and sharps (#). When the piano keyboard was invented, they made the white keys play natural notes and black keys play accidentals (flats and sharps).
As Western music developed, people became fonder and fonder of major keys (the “happier” sounding keys, to simplify things). This created a problem, as the simplest major key to only use natural notes was the C major key, which starts at C. The notes return to alphabetical order in the natural minor key, as the A minor scale plays as A-B-C-D-E-F-G-A.
