The order of the key signatures – flats

If one, and only one, flat appears in the key signature, it is always B flat.

If two flats appear in the key signature then it is always B and an E flat.

If three flats appear in the key signature then it is always B, E and A flat.

If four flats appear in the key signature then it is always B, E, A and D flat.

If five flats appear in the key signature then it is always  B, E, A, D and G flat.

If six flats appear in the key signature then it is always B, E, A, D, G and C flat.

If seven flats appear in the key signature then it is always B, E, A, D, G, C and F flat.

The order of the flats are always
B E A D G C F