In music theory, flat is a musical sign from the family of symbols called accidentals.
Here's what a flat looks like:
What does a flat do?
The flat in front of a music note, lowers the sound of the note by a half step (semitone ).
Little reminder of the half step and whole step concept:
The white keys on the piano keyboard correspond to the notes of the C Major scale, that are: C D E F G A B. The smallest distance between two notes is a half step, and in the C major scale, there is one whole step between two consecutive notes except between E and F and between B and C where there is only one half step. And off course, one whole step equals two half steps.
You will notice that there are black keys between the keys C and D, D and E, F and G, G and A, A and B. We will be able to play these black keys with accidentals and in particular with flat notes (notes lowered by one half step).
Here are all flat notes (this is the descending chromatic scale):
You will also notice that there are no black keys between the E and F notes and between the B and C notes, because as you just understood there is only one half step between E and F and between B and C. This does not mean that it is impossible to play an F flat or an C flat, but we will see it later.
The position of the flat: The flat can be placed in front of a note (it is an accidental) or next to the clef in the key signature.
With letters, we write "E flat" or "E♭" for short. But on a staff (stave ), the flat, like all the other accidentals, is written before the note, and here is a B flat:
And here are flats in the key signature of D flat major scale:
This key signature with five flats tells you that B, E, A, D, and G are lowered by one half step:
F flat? C flat?
We have learn that there is only one half step between E and F, and that flats lowers the sound by a half step, so a F flat is played with the E key on a piano keyboard. And of course it is the same for C flat which is played with the B key of the piano.
Don't worry, the double flat concept is very simple, here's a double flat:
The double flat lowers the sound of the note in front of which it is writen by two half steps (semitones), that is to say, it lowers the sound by one whole step (tone).
The double flat can be found in some scales, such as in the F flat major scale.
Don't worry, triple flats do not exist, but half flats do exist! Quarter tone accidentals are not frequently used and are used in contemporary music or to transcribe the melodies of Western music.
Half flats lowers a note by a quarter tone and can be written in different ways:
Three-quarter-tone flat (or flat and a half) lowers a note by three quarter tones and can be written in different ways:
Major scales with flats
Here are all major scales with flats:
1♭: F major scale
F G A B♭ C D E F
2♭: B flat major scale
B♭ C D E♭ F G A B♭
3♭: E flat major scale
E♭ F G A♭ B♭ C D E♭
4♭: A flat major scale
A♭ B♭ C D♭ E♭ F G A♭
5♭: D flat major scale
D♭ E♭ F G♭ A♭ B♭ C D♭
6♭: G flat major scale
G♭ A♭ B♭ C♭ D♭ E♭ F G♭
7♭: C flat major scale
C♭ D♭ E♭ F♭ G♭ A♭ B♭ C♭
Examples of flats in famous works
Frédéric Chopin's Nocturne opus 9 n°1 has a key signature with 5 flats:
(Royalty free sample performed by pianist Samson François in 1964, Creative Commons Zero 1.0 license, source)
In this excerpt, composed in the main key of B flat minor you will notice the use of double flats (measure 7), the use of the uncommon 6/4 time signature, and the use of tuplets (measures 3, 4 and 11) and eighth note triplets (measure 12).