What is a Key in Music? A Complete Guide

When playing or listening to music, you might hear someone say that a song is in a certain key, for example “This is in the key of G Major”. But what exactly does this mean? And how does a key relate to notes and scales and chords? 

This post will define what a key is in music, and how we can figure it out by looking at a written piece of music. 

What is a Key? 

In music, a key is the main group of pitches, or notes, that form the harmonic foundation of a piece of music.

The main pitches used in a song are usually all from one particular scale, and this is where we name the song’s key from.

For example, if a song only uses notes from the C Major scale, it is likely that the song is “in the key of C Major”.

This would mean no sharps or flats – C, D, E, F, G, A, B, C.

C major

However, the A minor scale also has no sharps or flats in it, so if we read a piece of music with no sharps or flats indicated, it might also be in the key of A minor.

A minor

How to Work Out a Song’s Key

There are always two keys that share the same key signature.

The key signature is what you see at the beginning of a piece of music right after the clef sign.

And right before the time signature.

Here is a key signature with five sharps (#), which is typically used for B Major.

B major

Notice how the five sharps are right in between the clef and the time signature.

However, this key signature could also be used to indicate a key of G# minor.

There is always a major and minor key for each key signature.

These are called relative keys, and they are always three semitones apart (the minor key is three semitones below the relative major key).

So, how do we find out whether or not a key is major or minor?

Usually the song or piece will start and end with a tonic chord, which is the chord that corresponds to the key of the song.

The note the tonic chord is based on (for example, a D Major chord is based on the note D) is just called the tonic

The tonic chord is also thought of as the home chord, and the song usually feels most normal when this chord or note is played.

So, in order to figure out what key a piece is in, we look for the tonic chord and note. 

In this example, the key signature says B Maj/G# min, and the melody begins and ends on a B, so we can assume the piece is “in the key of B Maj”.

If it starts and/or ends on G#, then it would be in G# minor, like in this next example: 

For some real examples, here are two piano sonatas by Haydn.

They both have the same key signature – one sharp – but one is in a major key and the other in a minor key. 

This one is in G Major, and you can notice right from the beginning because it starts (after the pickup) with a G note at the beginning of the first full bar.

Haydn’s ‘Sonata In G Major, Hob.XCI:27’

This next one is in E minor, and it starts immediately with an E minor arpeggio (E – G – B – E) in the bass clef to let you know that E is the tonic.

Haydn’s ‘Piano Sonata in E, No.53, Hob.XVI/34’

Here are two more examples – see if you can figure out what the key is for each piece.

The key signature is the same in both; it has one Flat.

One flat means either the key of F Major or D minor, so which piece is in the key of F Maj and which is in the key of D min? 

The first one is a Bach piano sonata in D minor. You can tell because the first notes played are a D minor chord – D, F, A. 

The second is a Mozart piano sonata in F Major.

The right hand melody starts by playing F – A – C, which makes an F Major chord, and the left hand accompanies it with the same notes.  

In Summary

This article hopefully has helped you learn what a key is in music.

To figure it out for a song that you are listening to on the radio or are learning to play, first check the key signature, and then try to find the tonic note or chord.

Sometimes songs can change keys, or modulate, so be on the lookout for those.

If you have any questions or comments please let us know below! 

Samuel Chase

Samuel Chase

Samuel Chase has been playing music since he was 5 years old, and teaching music since he was 13. He has a PhD in Music from the University of Surrey, and he has composed music that has been played in three different countries. He is currently working as a film composer and writing a book on film music.

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