flats lowering a note

Notes that are lowered by half a step (one semitone) are considered "flat".

Indicate a flat note by placing a _ (shift-minus) before the note name.

play "E _B G B"

Once a note is indicated as flat, that note will continue to be played flat until the end of the measure or line.

Indicate a natural (unflat) note by using = before the note name:

play "_B =B C' B"

Occassionally a double-flat is needed, and it can be written as __.

Read about key signatures if you are writing music with many flats.