Flat key signature

Lowers by a semitone the pitch of notes on the corresponding line or space, and all octaves thereof, thus defining the prevailing major or minor key. Different keys are defined by the number of flats in the key signature, starting with the leftmost, i.e., B?, and proceeding to the right; for example, if only the first two flats are used, the key is B? major/G minor, and all B's and E's are "flattened", i.e. lowered to B? and E?.