As nicely described in Wikipedia: In music, an accidental is a note whose pitch (or pitch class) is not a member of the scale or mode indicated by the most recently applied key signature.

In musical notation, the sharp (♯), flat (♭), and natural (♮) symbols, among others, are used to mark such notes, and those symbols may themselves be called accidentals. In the measure (bar) in which it appears, an accidental sign raises or lowers the immediately following note (and any repetition of it in the bar) from its normal pitch, overriding sharps or flats (or their absence) in the key signature. One occasionally sees double sharps or flats, which raise or lower the indicated note by a whole tone. Accidentals apply within the measure and octave in which they appear, unless canceled by another accidental sign, or tied into a following measure.

The modern accidental signs derive from the round and square small letter b used in Gregorian chant manuscripts to signify the two pitches of B, the only note that could be altered. The round b became the flat sign, while the square b diverged into the sharp and natural signs.

Understanding Steps and Accidentals


How to handle steps and accidentals when you play Mandolin

Distances between notes are typically referred to as half steps and whole-steps. In music theory you may find them also mentioned as semitones and tones.
A half step on the mandolin is the distance from one fret on the fret board to the next adjacent fret.
half step on the mandolin

A whole step is the same distance as two half steps.
whole step

As explained above, the flat lowers a note by a half step, while the sharp raises a note by half step.
So, when you you are reading sheet music, if you notice a sharp symbol just before the note name, you should play one fret higher than the named note.
Accordingly, when you see the flat symbol it means you need to play one fret lower. Simple, right?

All Basic articles in the Mandolin Theory series