Accidentals

An accidental is a symbol that raises or lowers the pitch of a note usually by a half-step. The most common ones are the sharp (♯), flat (♭), and natural (♮) symbols.

Sharps
A sharp symbol raises the pitch of a note by a half step. It is also known as a diese in some countries and looks like a hashtag #. The order of the sharp in key signatures is F♯, C♯, G♯, D♯, A♯, E♯, and B#. Although the single sharp is the most common, a double sharp (𝄪) and triple sharps (𝄲) also exist.

Flats
The flat symbol lowers the pitch of a note by a half step. It is known as bemol in some countries and looks like a lowercase letter b.

Just like the sharp, the flat also has double flats (♭♭) and triple flats (♭♭♭). The triple flat is more common on the clarinet than any other instrument because of the clarinet's transposing.