Editing Key signature (section)

{{more citations needed|date=May 2025}}
{{short description|Set of musical alterations}}
{{For|use in cryptography|Key signature (cryptography)}}
{{Image frame|content=<score>{ \magnifyStaff #2 \omit Score.TimeSignature \key bes \major s^"" }</score>|width=200|caption=Key signature showing B{{Music|b}} and E{{Music|b}} (the key of [[B-flat major|B{{flat}} major]] or [[G minor]])}}
{{Image frame|content=<score>{ \magnifyStaff #2 \omit Score.TimeSignature \key d \major s^"" }</score>|width=200|caption=Key signature showing F{{Music|#}} and C{{Music|#}} (the key of [[D major]] or [[B minor]])}}

In [[Western culture|Western]] [[musical notation]], a '''key signature''' is a set of [[Sharp (music)|sharp]] ({{Music|sharp}}), [[Flat (music)|flat]] ({{Music|b}}), or rarely, [[Natural (music)|natural]] ({{Music|natural}}) symbols placed on the [[staff (music)|staff]] at the beginning of a section of music. The initial key signature in a piece is placed immediately after the [[clef]] at the beginning of the first line. If the piece contains a section in a different [[Key (music)|key]], the new key signature is placed at the beginning of that section.

In a key signature, a sharp or flat symbol on a line or space of the staff indicates that the [[note (music)|note]] represented by that line or space is to be played a [[semitone]] higher (sharp) or lower (flat) than it would otherwise be played. This applies through the rest of the piece or until another key signature appears. Each symbol applies to comparable notes in all [[octave]]s—for example, a flat on the fourth space  of the [[Clef#Treble clef|treble staff]] (as in the diagram) indicates that all notes notated as [[E (musical note)|Es]] are played as [[E♭ (musical note)|E-flats]], including those on the bottom line of the staff.

Most of this article addresses key signatures that represent the [[Diatonic and chromatic|diatonic]] keys of Western music. These contain either flats or sharps, but not both, and the different key signatures add flats or sharps according to the order shown in the [[circle of fifths]].

Each [[Major scale|major]] and [[Minor scale|minor]] [[key (music)|key]] has an associated key signature, showing up to seven flats or seven sharps, that indicates the notes used in its scale. Music was sometimes notated with a key signature that did not match its key in this way—this can be seen in some [[Baroque music|Baroque]] pieces,<ref>Schulenberg, David. ''Music of the Baroque''. New York: Oxford University Press, 2001. [https://books.google.com/books?id=P4x3aKvOQWAC&pg=PA72&vq=%22the+key+of+a+baroque+work%22&source=gbs_search_r&cad=1_1&sig=ACfU3U3x7lhOyyIiT-O_HsTZlZxw8FrkSA p. 72.]. "(…) to determine the key of a Baroque work one must always analyze its tonal structure rather than rely on the key signature."</ref> or transcriptions of traditional [[Mode (music)|modal]] [[Folk music|folk]] tunes.<ref>Cooper, David. ''The Petrie Collection of the Ancient Music of Ireland''. Cork: Cork University Press, 2005. [https://books.google.com/books?id=PzrfncNnr4cC&pg=PA25-IA13&vq=%22In+a+few+cases+Petrie+has+given+what+is+clearly+a+modal+melody+a+key+signature+which+suggests+that+it+is+actally+in+a+minor+key%22&source=gbs_search_r&cad=1_1&sig=ACfU3U0BqVPTuQnnaH32iLpCvrYnyT143Q p. 22]. "In a few cases Petrie has given what is clearly a modal melody a key signature which suggests that it is actually in a minor key. For example, ''Banish Misfortune'' is presented in D minor, although it is clearly in the Dorian mode."</ref>