Let’s look at the key of F major. F major features a flat note in its key signature. Flattening a note works in much the same way as sharpening a note, except this time you’ll be lowering a note by a semitone. The special flat note in F major is B-flat. That means playing the black key between notes A and B.
The reason why we need to add sharps and flats to certain keys is because there is a formula that all major scales follow. Without this formula all of your scales will sound slightly off in one way or another. Let’s take a look at how this formula works to give us that major scale sound. We can break any scale down into a series of full-tones and semitones. You’ve already learned about semitones via sharps and flats. A full-tone step is two piano keys apart. For example, the jump from E to F is a semitone jump, as there's not black key in between..
A major scale is mostly made up of fulltones with two big exceptions. There are semitone jumps between notes 3-4 and notes 7-8 of every major scale. If you look closely at the pattern of the keyboard from starting on F, you can see why we need to flatten the fourth note, B in order for the scale to work within that formula. As long as you remember those two semitone ‘jump-points’ you can play that major scale pattern in any key!
Because of this B-flat note, there’s a slight change in the fingering pattern needed in order to play the F major scale accurately. All that you need to change is instead of playing 1-2-3 and then thumbtucking to reset your hands on the 4th note, this time you’ll play 1-2-3-4 and THEN thumbtuck to reset your hand on the 5th note.