There are 12 notes or tones in western music. Each note is 1/2 step apart, and together in sequence they make up the Chromatic Scale (blue and green notes below). These notes are named after the alphabet letters ABCDEFG.
Between each named note (exceptions are BC and EF) there is note designated as sharp or flat. Whether that note is a sharp or a flat depends on the key in which the note lives. For example, the note between A and B is Bb in the key of F, but it is A# in the keys of B, F# and C# (See: Circle of Fifths and Music Staff).
The sharps and flats chart below illustrates how a note can be both a sharp and a flat. The only intervals without a sharp or flat between them are BC and EF.
The guitar neck below illustrates the Chromatic Scale, seen as though the guitar were sitting on your lap face up.
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