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What a key signature does is plain but very easily misread:
It sets defaults — a group of sharps or flats is written once after the clef, and those notes thereafter appear automatically in their altered form, without an accidental on each.
It is a labour-saving device, not a label for a key. That distinction is the heart of this entry.
The order of sharps and flats
| Type | Order | How to remember |
|---|---|---|
| sharps | F · C · G · D · A · E · B | the order you get walking clockwise round the circle of fifths |
| flats | B · E · A · D · G · C · F | the reverse of the sharp order |
Why that order: it is exactly the sequence of clockwise (sharps) and anticlockwise (flats) steps from C on the circle of fifths. So "why is the order of sharps F–C–G–D–A–E–B" is answered on the circle.
⭐ The key insight: a signature does not name a key
The most important point here:
| Signature | Keys it can indicate |
|---|---|
| no sharps or flats | C major, or A minor, or any mode on C |
| one sharp (F♯) | G major or E minor |
| two flats | B♭ major or G minor |
So a signature tells you which notes are in use, not which key you are in — for the key you must read the cadences and the central pitch (see tonal centre).
Which answers a common puzzle: "this piece has no sharps or flats, so it is C major?" Not necessarily — it may be A minor, or a mode.
How accidentals relate to the signature
| Key signature | Accidental | |
|---|---|---|
| Position | after the clef | before a note |
| Scope | the whole passage (until changed) | within its bar |
| Function | sets defaults | a one-off alteration |
"An accidental lasts only to the end of its bar" is a hard rule: if the same alteration is needed in the next bar, it must be written again. The rule puzzles beginners but has a clear reason: the bar is the basic unit of accent and harmony (see metre), so a sign's scope is divided the same way.
What happens when the signature changes
| Case | Practice |
|---|---|
| modulating to a new key | write the new signature at the start of a new system, sometimes cancelling the old at the end |
| cancelling temporarily | use naturals (♮) one by one |
| a brief tonicization | do not change the signature; use accidentals instead (see [[concept:accidentals |
That third choice is the practical heart of notation: accidentals for what is near, a new signature for what is far — so "the signature changed mid-piece" usually means a substantial modulation took place (see modulation).
Diagram: the signature as defaults
Listen: one set of notes, two centres
The same notes can serve a major or a minor key — the aural evidence that a signature does not name a key. This site cannot show a signature, but it can play two scales with different centres:
Common misconceptions
- "The key signature is the key." It says only which notes are in use. One signature serves a major key, its
- "No sharps or flats means C major." It may be A minor, or a mode on C (C Dorian, for instance).
- "Every sharp in the signature must be played each time." They are defaults: the notes appear altered
- "An accidental lasts." Only to the end of its bar. If the alteration is needed in the next bar, **write it
relative minor and various modes.
automatically, so no accidental is needed.
again**. :::
Listen in the library
All tracks from midicn-lib — inline badge is the license tier (C1 commercial / C2 non-commercial / C3 study only).
Sources
- 调号写在谱号之后并全局生效；升号顺序 F-C-G-D-A-E-B、降号顺序为其倒序，属记谱法通则
- 同一调号可对应大调及其关系小调（含各种调式），为通行乐理表述
Used for fact-checking only; all prose is original.