Chasmac is a semi-retired guitar teacher who has taught in various schools in London and elsewhere for over 30 years.
What's a Key in Music?
When we say a song or composition is in a particular key, it means it has been composed mainly from notes belonging to a particular major or minor scale. The songwriter or composer will also treat certain scale notes more importantly than others. Music in the key of C major, for example, will feature lots of C notes and C major chords at important places, such as at the start or ends of verses and choruses. Music in the key of C major will almost always have a C note or C major chord at the end of the song. Other notes and chords from the C major scale will typically 'lead up' to that key note C in such a way that the 'arriving home' effect is strong and obvious. Think of the last note of "Happy Birthday to YOU". That unmistakable feeling of finality isn't just because it's the last note, but because it's the key note of whatever key people have chosen to sing it in on any particular birthday. Composers and song writers are free to make the effect less obvious if that's what they want, but for a song like Happy Birthday, you can see the need for a strong 'home note' or tonal centre.
Similarly, if a song is in the key of C minor, the note C will again be treated as the 'home note', but the notes used in this song will mostly be from the scale of C minor (in its various natural, harmonic and melodic forms).
In standard notation, key signatures are groups of up to seven flats or seven sharps placed on the appropriate lines and spaces at the beginning of the music staff. Composers writing in a particular key will make use of the one that corresponds with the number of flats or sharps in their chosen key.
Music in the key of A flat major, for example, will have four flat notes used repeatedly throughout the music (Bb, Eb, Ab & Db) . Rather than having to show the flat symbols (b) each time these notes occur in the music, the key signature of the same four flats is used instead. It makes for a much cleaner page of written music. Music in the key of F minor, has the same four flats and so the same key signature would be used.
Something that all music students have to know is how keys are related. Being related refers to the number of notes that they (i.e., their scales) have in common.
The key of C major, for example, has no sharps or flats because the scale of C major has no sharps or flats. Any sharps or flats that appear in any music in the key of C major will be foreign, out of key, notes brought in for a special effect or some other purpose.
The key of A minor also has no sharps or flats because the A natural minor scale has no sharps or flats. The A harmonic and melodic minor scales can also provide a couple of altered notes, F# and G#. These modifications are so common, especially in classical music, that they aren't considered foreign notes as such (although they aren't included in any key signature). Only the natural minor scale is used in determining the principal notes of minor keys.
So, as both keys of C major and A minor have no sharps or flats, it means ALL their notes are the same, CDEFGAB and ABCDEFG. That being the case, A minor is called the relative minor key of C major, and C major is called the relative major key of A minor.
Keep in mind that, although they have exactly the same principal notes and are closely related in some ways, in another equally important way, they are fundamentally different because they have different key notes or tonics. All of the notes will relate to the different tonics, A & C in different ways which creates the distinctively different character we can often hear in songs in minor keys and major keys. (See major and minor qualities, below, for more on this.)
The Circle of Fifths
The circle of fifths diagram below shows the relationships between keys in music separated by intervals of a perfect 5th. It lets us see at a glance keys that are closely related or distantly related or anywhere in between in terms of the number of notes that they have in common.
Circle of fifths
In the circle of fifths chart, major keys are arranged around the outside, and their relative minors are shown in the inside.
As you can see, for every major key there's a relative minor key that corresponds with the 6th note of the major scale (A is the 6th note of the C major scale) and for every minor key, there's a relative major key that corresponds with the 3rd note of the minor scale (C is the 3rd note of the A minor scale).
All major keys have a unique arrangement and number of sharps or flats, as do all minor keys. Arranging them spaced by intervals of 5ths in a clockwise direction reveals their close relationship to their immediate neighbours. Each key will only differ from both of its neighbours by a single sharp or single flat note.
Circle of fourths
The circle works equally well if you look at it from the opposite direction, i.e., in an anti-clockwise direction. In that case the keys are separated by intervals of a perfect 4th, which being an upside down (inverted) perfect 5th, comes to exactly the same thing. For that reason, the circle is also known (albeit less commonly) as the circle of fourths.
Looking at the outer ring of the circle, you can see at the 6 o' clock position, the key of F sharp major, with 6 sharps, is in the same position as G flat major with 6 flats. These two keys (and their scales) sound exactly the same and are said to be enharmonically equivalent, so it's convenient to switch over to the other at that point when going round the circle in either direction. The same applies to the relative minor keys of D sharp minor and E flat minor.
As you can see, by going around clockwise, one more sharp is added each time. By the time we get to F sharp major at the bottom, we've amassed 6 sharps. We can't keep adding sharps indefinitely or we'll soon end up with more sharps than notes (double sharps). We can go one more step to the 7 o'clock position which will give us the keys of C sharp major or A sharp minor (not shown on the chart), with seven sharps - one for every note, but we can't go any further without getting into keys that have more sharps than notes.
What we can do, though, is switch names. If we switch over to the identical sounding (enharmonically equivalent) key of G flat major with 6 flats instead of F sharp major with 6 sharps, we can continue in the same clockwise direction, but this time we'll be subtracting flats instead of adding sharps (which is essentially the same thing). One by one, each key a fifth higher will have one less flat, instead of one more sharp. So instead of having the 7 sharps of C sharp major at the 7 o'clock position, we can have the slightly simpler enharmonic equivalent key of D flat major with only five flats. By thinking in terms of decreasing flats instead of increasing sharps, we can continue in this direction until we eventually complete the circle and arrive back at C major, with no flats or sharps.
Note 1* Coming from the opposite direction follows exactly the same principle. The amount of flats increases one by one with the seven flats of C flat major (not shown on this particular chart) being the maximum. Switching over at the 6 o' clock position from G flat major to its enharmonic equivalent, F sharp major, means the next key, a fourth higher (or a 5th lower if you prefer) will give us the key of B major with five sharps instead of C flat major with seven flats. Continuing in this direction will reduce the amount of sharps one by one, until we again arrive back at C major with no sharps or flats.
Note 2* Exactly the same process applies to the inner ring of minor keys. The minor keys at the 6 0'clock position (D sharp minor and E flat minor) have six sharps and six flats respectively, and sound exactly the same, that is, they are enharmonically equivalent.
Major and minor key qualities
A song in the key of D minor, (for example), will have a very different character to a song in D major. Both keys have the same 'home note' or tonic, D, but some of their scale notes are different. This means that those notes will form different intervals with the tonic, D, and create a distinctive (minor) character.
Minor keys are great for quiet reflective beauty, (e.g., the slow movement of Beethoven's 'Moonlight Sonata'). They're also ideal for suspense, tragedy, 'sin and misery while their guitar gently weeps' kinds of song, but they're pretty useless at conveying joy, triumph, celebration, etc. You couldn't really have a minor key version of Happy Birthday and expect it to convey the same good wishes as convincingly as the original major key version. That's what major keys do best. Songs in major keys can convey a stronger, brighter, positive mood quite easily, but, with a slow tempo, can be very sad and sombre too. (e.g., You were always on my mind).
Keys and key changes in music
Not all music is key based but the vast majority of Western music from around the 17th century to the present day, including classical, rock, pop, folk and jazz, is key based.
Music that establishes the key and tonal centre through functional chord progressions is called tonal music and was a defining feature of Western classical music from around 1650 to 1900, the period known as the Common Practice Period (CPP). Later composers, such as Debussy, introduced non-tonal elements such as the whole tone scale as well as reviving pre-tonal musical modes.
Other key based music, pop, rock, blues and jazz, etc., has a lot in common with tonal music but is much looser in its approach to establishing the tonal centre. It's often called tonal music too, but more accurately is called 'centric' music. Blues music, for examples is centric, It has a tonal centre, but isn't strictly tonal as it includes out of key notes (blue notes) used as principal notes.
Key changes are a standard feature of most Western classical music and a common feature of jazz and more complex rock and pop styles. Even the simplest classical compositions usually include changes of key to and from the most closely related keys. Large scale symphonic works of the mid to later Common Practice Period typically have an increasingly complex arrangement of key changes. Most changes are still to closely related keys, but they also have development sections, where the composer explores lots of different, often very remote, keys before returning to the 'home key' of the whole composition.
The process of changing key within a song or composition is called modulation. Modulations can be long drawn out affairs, where the new key is established subtly through well-chosen chord progressions. Alternatively, they can be abrupt modulations, where the key changes without any preparation beforehand.
The process of briefly hinting at a new key, without actually going so far as to establish it fully, is called tonicisation.
The process of reproducing a piece of music in a new key (or keys) by changing the pitch of every note equally is called transposition.