Tonality is the system of hierarchy among pitches that sets up one main note, the tonic, around which all the other notes of a piece gravitate.
When a score is written in the key of C major, the notes and chords respect the distances of the C major scale, which acts as the point of greatest rest. The major mode is associated with a bright or stable sound; the minor mode, with a darker, more introspective one.
The articles below cover the circle of fifths and the role of key signatures, so you can tell in seconds which altered notes belong to each key.
What is a key?
In Western music, a key is the system of relationships established among a series of pitches, where one acts as a center of gravity or "resting note". This main note is called the tonic, and all the other notes of the piece revolve around it, creating tensions and resolutions.
If a song is in the key of C major, it mainly uses the notes of the C scale, and the C major chord is the final point of rest.
Difference between tone, scale and key
These three terms are often confused, but in analysis they mean different things:
- Tone: In Spanish, "tono" means the distance (interval) of two half steps, a whole step (UK: tone). In everyday speech it is sometimes used as a synonym for key ("this song is in another key").
- Scale: simply a ladder of notes in sequence (for example, C-D-E-F-G-A-B). It is the alphabet.
- Key: the complete ecosystem. It is not limited to playing the notes in order; it governs which chords are formed, which scales can be laid over them and how phrases are structured. It is the complete grammar.
The solfège names used in Spanish-, French- and Italian-speaking countries (Do, Re, Mi, Fa, Sol, La, Si) correspond to C, D, E, F, G, A, B.
Key signature: the ID of the key
To avoid drawing accidentals (sharps and flats) all the time throughout the score, they are grouped after the clef. This group is called the key signature. Each key signature tells the musician which key the piece is in without having to analyze the individual notes.
| Accidentals in the key signature | Main major key | Relative minor key |
|---|---|---|
| None (0) | C major | A minor |
| 1 sharp (F♯) | G major | E minor |
| 1 flat (B♭) | F major | D minor |
| 2 sharps (F♯, C♯) | D major | B minor |
The circle of fifths
To memorize all the key signatures, a visual tool called the circle of fifths is used. This diagram arranges the keys so that those sharing almost the same notes fall in neighboring positions (for example, C major and G major). It is essential for understanding modulations (changes of key in the middle of, or throughout, a song).
The full explanation, with a chart of the circle of fifths and a table of the key signatures of all keys, is on its own page.
A historical note: Before well temperaments and, later, equal temperament became standard, not every key could be played on keyboard instruments, because the older tuning systems left some intervals badly out of tune (the so-called "wolf" intervals). Bach's Well-Tempered Clavier (1722) shows all 24 keys (12 major and 12 minor) in use on a keyboard tuned with a well temperament; equal temperament, which divides the octave into twelve identical half steps, later became the modern standard.
To practice identifying keys and key signatures, there are interactive key exercises.