A flat lowers the note by a semitone.
As a key signature, a single flat indicates the key of F, meaning your B should be flat. Two flats are E and B, meaning the key is therefore B flat. B, E and A flat indicate the key of E flat ... and so on.
There are three flats in the key signature of E flat major
Five flats:B-flat, E-flat, A-flat, D-flat, G-flat.
B-flat
What is a key signature? A key signature is the number of sharps or flats in a piece of music. For example C Major has no sharps G Major has 1 Sharp F Major has 1 Flat B flat
A key signature
The key signature for F major has one flat (B flat) , while the key signature for F minor has four flats (B flat, E flat, A flat, and D flat).
The key signature that has B flat, A flat, E flat and D flat is Concert A flat Major.
That is the signature for the key of Db. Or, it's relative minor, Bb minor.
B flat, E flat, A flat, D flat, G flat, C flat, F flat. It's the opposite order to that of the sharps in a key signature.
There are three flats in the key signature of E flat major
B flat, E flat, A flat, D flat
Assuming the key signature has only one flat (B Flat) then the key is either F Major, or D minor.
Five flats:B-flat, E-flat, A-flat, D-flat, G-flat.
When working with flats, the second to last flat is the key signature. The exception here being when only one flat is in the key signature - this would be the key of 'F.' The order of flats are as follows: B-flat, E-flat, A-flat, D-flat, G-flat... etc.
B flat, E flat, A flat, D flat, G flat.
That is C Minor
The key signature with only a B-flat is either F major or D minor.