Notation and the Measure of Sound · Lesson 4
Key Signatures and Accidentals
about 15 min
What this lesson is for
A key signature is a declaration parked at the left of the staff saying which notes this piece alters. The order sharps and flats appear in comes from the interval of a fifth, and reading a tonic back out of a signature follows a fixed procedure. Accidentals and their scope round the lesson out.
- Explain why key signatures exist at all - to avoid repeating the same accidental
- Recite the order sharps and flats are added in, and trace that order to the circle of fifths
- Identify the major tonic from a key signature
- State the scope of an accidental - same measure, same octave
In this lesson
The staff and a clef between them can only write the seven natural letter names. Writing anything else requires an accidental.
There are five of them.
| Sign | Name | Effect |
|---|---|---|
| ♯ | sharp | raises by a half step |
| ♭ | flat | lowers by a half step |
| ♮ | natural | cancels a previous alteration |
| × (♯♯) | double sharp | raises by a whole step |
| ♭♭ | double flat | lowers by a whole step |
The point that matters most here is that an accidental does not move the note on the staff. C# occupies the same line or space as C. This is the property established in the first lesson coming back: the staff maps letter names, not physical distances.
Key signatures exist so you stop rewriting the same sign
Suppose a piece is in G major. The G major scale is G A B C D E F# G, which means essentially every F in the piece will be an F#.
Marking each one individually would bury the page in symbols. So the alteration gets declared once, at the left edge of the staff. That is a key signature: a standing instruction to read every F in this piece as F#.
The payoff is that notes belonging to the key can now be written plain. And the flip side is just as valuable: only notes from outside the key need an accidental, so harmonically special events stand out visually the moment they occur. For a reader that is an enormous advantage.
Check
In a piece whose key signature has three flats, a B appears with nothing written beside it. What pitch is it?
The order of sharps and flats is not arbitrary
The accidentals in a key signature are not arranged by taste. They follow a fixed order, and that order has the interval of a fifth built into it.
Sharps: F → C → G → D → A → E → BSet the two lists side by side and the first thing you notice is that each is the other reversed. The second is that consecutive names in the sharp order are all a perfect fifth apart (F to C to G to D to A to E to B). The flat order runs by descending fifths, which is the same as ascending fourths.
The reason is structural. Every time you build a major scale a fifth higher, exactly one more note needs altering. C major needs none; a fifth up, G major needs F#; another fifth up, D major adds C#. Moving around the circle of fifths and adding accidentals to the signature are the same motion described two ways.
That structure gets treated head-on as the circle of fifths in Unit 2, in the lesson on key relationships. For now it is enough to hold onto the fact that the order comes from fifths.
What actually changes when the key signature changes
Check: All three are the same shape — a major scale. Only the starting pitch differs, and the key signature is what preserves the shape at each new starting point. Confirm that all three sound like the same bright scale while sitting at different heights.
zero accidentals
Reading the key out of the signature
Being able to name the key at a glance changes how fast you read. For major keys there is a mechanical rule.
Sharp keys: the tonic is one letter name above the last sharp.
- one sharp (F#) → one above F = G major
- three sharps (last is G#) → one above G = A major
- five sharps (last is A#) → one above A = B major
Flat keys: the tonic is the name of the second-to-last flat.
- two flats (Bb, Eb) → second-to-last is Bb = B-flat major
- four flats (Bb, Eb, Ab, Db) → A-flat major
- one flat is the exception: F major, which simply has to be memorized
| Signature | Major | Relative minor |
|---|---|---|
| none | C | A m |
| ♯1 | G | E m |
| ♯2 | D | B m |
| ♯3 | A | F♯ m |
| ♯4 | E | C♯ m |
| ♯5 | B | G♯ m |
| ♯6 | F♯ | D♯ m |
| ♭1 | F | D m |
| ♭2 | B♭ | G m |
| ♭3 | E♭ | C m |
| ♭4 | A♭ | F m |
| ♭5 | D♭ | B♭ m |
| ♭6 | G♭ | E♭ m |
One caution belongs here. A single key signature stands for two keys: a major key and its relative minor, a minor third below. They draw on the same set of pitches, so the signature by itself cannot tell them apart.
Distinguishing them means looking at the music. Two practical clues:
- which note or chord the piece begins and ends on (it lands on the tonic)
- in minor, the seventh degree is usually raised, and shows up as an accidental
The second is the stronger signal. A score in C minor is full of B naturals that the key signature never declared. That raised seventh is the minor-key leading tone, and it becomes a central character in the harmony units ahead.
Check
A key signature has two flats (Bb, Eb). Which minor key does it represent?
How far an accidental reaches
An accidental is what you write for a note outside the key signature, and its scope is precisely defined.
An accidental applies within its own measure, to that exact pitch only.Concretely:
- it expires at the barline (the next measure starts clean)
- it does not carry to other octaves (sharpening F4 leaves F5 alone)
- within the same measure at the same pitch, it keeps applying without being rewritten
Real scores often add a courtesy accidental to head off misreadings — a parenthesized ♮ on the note after the barline, for instance, where the previous alteration has just lapsed. Nothing in the rules requires it; it exists purely to keep players out of trouble.
Check
Beat 1 of a measure has an E with a flat. On beat 3 of the same measure, an E appears an octave higher. What is that note?
Why double sharps and double flats have to exist
It is tempting to object that C double sharp is just D, so one may as well write D. But double accidentals are not a notational quirk; they are forced by musical structure.
Take the harmonic minor scale of G# minor, which raises the seventh degree. That seventh degree is F#, so raising it gives F×, F double sharp. Writing G instead is not an option: the seventh degree has to be some kind of F, because calling it G would collide with the eighth degree, the tonic G#, and leave the scale with two Gs and no F.
That principle — that a scale degree is determined by its letter — is the whole subject of the next lesson.
See the signatures laid out on the circle of fifths Each step clockwise adds a sharp; each step counterclockwise adds a flat. The order of the key signatures turns out to be the motion of fifths itself Circle of Fifths → Play a scale with three flats in the signature This is E-flat major. Watching which black keys it uses tells you exactly which three notes those flats refer to Scale Dictionary →Exercises
Every answer here is cross-checked against the theory engine. Work them out before you grade.
- 1
How many sharps are in the key signature of G major?
- 2
How many flats are in the key signature of E-flat major?
- 3
Write out the first four sharps in the order they are added, as letter names separated by single spaces.
- 4
A key signature has four sharps. What is the major tonic?
- 5
In a measure of 4/4, beat 2 carries an F with a sharp written on it. On beat 4 of the same measure, an F appears at the same pitch. What is that F?
Before you move on
Check that you can explain each point above in your own words.