In music, in English sharp – eqv. dièse (from French) or diesis (from Greek δίεσις)^{[lower-alpha 1]} – means higher in pitch. The sharp symbol, ♯, indicates that the note to which the symbol is applied is played one semitone higher. The opposite of sharp is flat, indicating a lowering of pitch. The ♯ symbol derives from a square form of the letter b (see History of notation of accidentals for more information).

The sharp symbol is used in key signatures or as an accidental applied to a single note. The staff below has a key signature with three sharps (A major or its relative minor, F♯ minor). The sharp symbol placed on the note indicates that it is an A♯ instead of an A♮.

In twelve-tone equal temperament tuning (the predominant system of tuning in Western music), raising a note's pitch by a semitone results in a note that is enharmonically equivalent to the adjacent named note. In this system, A♯ and B♭ are considered to be equivalent. There are other tuning systems in which this is not the case, however.

In a key signature, sharps or flats are placed to the right of the clef. The pitches indicated apply in every octave.

The order of sharps in key signature notation is F♯, C♯, G♯, D♯, A♯, E♯, B♯. Starting with no sharps or flats (C major), adding the first sharp (F♯) indicates G major, adding the next (C♯) indicates D major, and so on through the circle of fifths.

Some keys (such as C♯ major with seven sharps) may be written as an enharmonically equivalent key (D♭ major with five flats in this case). In rare cases, the sharp keys may be extended further, G♯ → D♯ → A♯ → E♯ → B♯ → F → C, requiring double sharps in the key signature: F, C, G, D, A, E, B. These are called theoretical key signatures. This principle applies similarly to the flat keys.

When used as an accidental, the sharp symbol is placed to the left of the note head.

Accidentals apply to the note on which they are placed, and to subsequent similar notes in the same measure. In modern notation they do not apply to notes in other octaves, but this was not always the convention. If a sharp is used as an accidental, it can be cancelled on a subsequent similar note in the measure by using a flat (♭) or a natural (♮).

A double sharp is indicated by the symbol and raises a note by two semitones, or one whole tone.

Historically, a double sharp was sometimes written , or .^[1]

Less often (in microtonal music notation, for example) other types of sharps may be used: A half sharp, or demisharp, or quarter tone () raises a note by approximately a quarter tone = 50 cents (Play^ⓘ), and may be marked with various symbols, often including , sometimes ++ instead. A sharp-and-a-half, three-quarter-tone sharp, or sesquisharp, raises a note by three quarter tones = 150 cents (Play^ⓘ) and may be denoted .

Although very uncommon, a triple sharp (♯) can sometimes be found. It raises a note by three semitones or one whole tone and one semitone.^[2][3]

The sharp symbol (♯) resembles the number (hash) sign (#), in that both have two intersecting sets of parallel double lines. While the number sign may have a pair of horizontal lines, the sharp sign has a pair of slanted lines that rise from left to right instead, to avoid obscuring the staff lines. The other set of parallel lines are vertical in the sharp sign, while the number sign (#) may have slanted lines instead.^{[citation needed]} It is also etymologically independent from the number sign.

Likewise, while the double-sharp sign resembles a bold-face lower-case x it needs to be typographically distinct.

In Unicode, assigned sharp signs are as follows:

• U+266F ♯ MUSIC SHARP SIGN (♯)
• U+1D12A 𝄪 MUSICAL SYMBOL DOUBLE SHARP
• U+1D130 𝄰 MUSICAL SYMBOL SHARP UP
• U+1D131 𝄱 MUSICAL SYMBOL SHARP DOWN
• U+1D132 𝄲 MUSICAL SYMBOL QUARTER TONE SHARP

• Historically, lowering a double sharp to a single sharp could be notated using a natural and sharp sign (♮♯) or vice-versa (♯♮) instead of the conventional sharp sign (♯), but the natural sign is often omitted in modern notation. The similar principle of the natural sign notation can apply when canceling a triple sharp or beyond.^[4] The combination ♮♯^[5] can be also written when changing a flat to a sharp.^[6]

• To allow extended just intonation, composer Ben Johnston uses a sharp to indicate a note is raised 70.6 cents (ratio 25:24).^[7]
• In environments where the symbol is not supported, or in specific text notation, a double sharp is sometimes written using two single sharp signs (♯♯) or a lower-case letter x. Likewise, a triple sharp can be written as ♯♯♯.

• Flat (music)
• Electronic tuner