Keys and Key Signatures

Keys

In music, a key describes the tonal centre of a piece and the set of notes and chords primarily built around that centre. The tonal centre is called the tonic. It is the note that feels the most stable and resolved, often giving the sense of musical “home”.

When a piece is in a particular key, the notes it uses will usually come mostly from the scale built on that tonic. For example, if a piece is in the key of C major, it will largely use the notes of the C major scale:

C D E F G A B

From these notes we can also build the set of diatonic chords of the key. These are the chords we get when we harmonise the major scale by stacking thirds.

Diatonic Triads in C Major

The Roman numerals show each chord’s position relative to the tonic. Uppercase numerals represent major chords, lowercase numerals represent minor chords, and the small circle symbol (°) indicates a diminished chord.

Because the same harmonic pattern occurs in every major key, the Roman numerals stay the same while the actual chord names change depending on the tonic.

Examples in Three Different Keys

Minor Keys and Relative Keys

Keys can be described as either major or minor. This depends on the type of scale that forms the foundation of the key and its tonal centre.

Just as the major scale forms the basis of major keys, the minor scale forms the basis of minor keys. When a piece is in a minor key, the notes and chords are mainly drawn from the minor scale built on that tonic.

For example, the key of A minor is based on the A natural minor scale:

A B C D E F G

This means A is heard as the tonal centre, even though the notes are the same as C major.

This leads to an important relationship between major and minor keys called relative keys. A relative major and relative minor use exactly the same notes, but they have different tonics.

For example:

C major: C D E F G A B

A minor: A B C D E F G

Both keys use the same seven notes, but C major sounds centred around C, while A minor sounds centred around A.

Every major key has a relative minor, and every minor key has a relative major. The relative minor of a major key is found on the 6th degree of the major scale. The relative major of a minor key is found on the 3rd degree of the minor scale.

So, although relative keys share the same notes, they are still heard as different keys because the tonal centre is different.

Key Signatures

In our earlier lesson on the major scale, we saw that the interval pattern of the scale stays the same regardless of the note we start on. The distances between the notes always follow the same structure:

Whole tone – Whole tone – Semitone – Whole tone – Whole tone – Whole tone – Semitone

Because this pattern stays fixed, starting the scale on different notes creates different collections of seven notes. Each starting point produces its own version of the major scale.

If we logically apply this pattern to all twelve pitch classes, we end up with twelve distinct sounding major keys. Each of these keys follows the same interval structure but begins on a different tonic.

However, when these note collections are used to form complete keys in written music, we quickly run into a practical problem.

For example, compare the C major and G major scales:

Why we use Key Signatures

Notice how in the previous example the notes are almost identical, except that the F in C major becomes F♯ in G major.

When music is written on the staff, the lines and spaces represent the natural notes A to G. Sharps and flats are shown using accidentals placed next to the notes that are altered.

If we were writing a piece in G major, every F would need to be written as F♯. This means a sharp symbol would have to appear next to every F throughout the piece, which makes the music slightly more awkward to read and write.

Instead, we use a system called a key signature.

A key signature places the required sharps or flats at the beginning of the staff. This tells the performer which notes are consistently altered throughout the piece, so the accidental does not need to be written every time the note appears.

For example, the key of G major uses one sharp: F♯. Rather than writing a sharp next to every F, the key signature simply places a sharp on the correct line of the staff at the start.

The diagram below shows the key signature for G Major in standard musical notation.

Key Signature of G Major:

Organising Key Signatures

If we organise the major keys by the number of sharps in their key signatures, a clear pattern begins to appear. Starting from C major, which has no sharps, each new key adds exactly one additional sharp.

For the first 6 keys, the order looks like this:

The diagrams below show how these key signatures appear on the staff.

Each new sharp appears in a fixed order, and each key adds the next sharp in that sequence.

Relationship Between the Sharps and the Tonic

There is also a useful relationship between the sharps in the key signature and the tonic of the key.

In sharp keys, the final sharp in the key signature sits a semitone below the tonic.

For example:

• In G major the only sharp is F♯, which sits a semitone below the tonic G.
• In D major the final sharp added is C♯, which sits a semitone below the tonic D.
• In A major the final sharp is G♯, which sits a semitone below the tonic A.

This small pattern allows musicians to quickly identify the key just by looking at the last sharp in the key signature.

A Pattern of Fifths

Another interesting pattern appears if we look at the tonics of these keys.

C → G → D → A → E → B

Each new key is a perfect fifth above the previous one.

This repeating movement by fifths leads us to one of the most important organising tools in music theory: the Circle of Fifths.

The circle of fifths shows how keys are related to each other and how key signatures grow as we move through the system. We will explore this idea in much more detail later.

Why Some Keys Use Flats Instead of Sharps

The next key after B major continues the same pattern and simply adds another sharp. For a while this system still works perfectly well. However, if we keep moving further in the same direction, we eventually reach a point where maintaining the correct scale pattern would require notes to be written with double sharps or with repeated note letters.

Because of this, musicians usually avoid writing certain keys using large numbers of sharps. Instead, those same pitches are written using flat key signatures, which are much easier to read and work with.

Lets look at F Major for example

Although this sounds the same, the sharp-only spelling repeats the letter A and skips the letter B entirely. Notation like this makes the structure of the scale harder to see, which is why flats are used instead in this case.

Let’s look at an even more extreme example.

The A♭ spelling keeps the musical alphabet in order and uses each letter name exactly once, which makes the scale easy to understand.

However now the notation becomes much more complicated. Several notes use unusual spellings such as B♯ and E♯, and the final note requires a double sharp (F𝄪).

For this reason, musicians usually prefer to write this key as A♭ major instead of G♯ major. Using flats keeps the notation simpler and preserves the clear alphabetical structure of the scale.

Keys with Enharmonic Equivalents

In some cases, a key can be written using either sharps or flats while producing the same pitches. When two notes or keys sound the same but are written differently, they are called enharmonic equivalents.

This happens because the sharp and flat systems eventually meet as we continue building keys. Instead of continuing with increasingly complex spellings, musicians often choose the enharmonic version that produces the clearest and most readable notation.

Some of the most common enharmonic key pairs are:

These pairs represent the same pitches, but they are written differently depending on the context and which spelling makes the music easier to read. Review the musical alphabet and enharmonic notes if the spellings are unfamiliar, then use the Circle of Fifths to organise the keys visually.