When you’re playing music, whether you are alone or with other musicians, knowing in what key you are is essential at all times. If you aren’t sure of which key a given song is, it might lead you to an awkward situation when it is your turn to play a solo.
There are songs that stay within the same key all the time, and others, like many jazz standards for example, might modulate to different keys throughout the music, which makes it absolutely necessary to be able to figure this out easily.
Want to learn more about music theory?
Check out our ultimate guide to music theory to find more jumping off points.
What is a Song’s Key?
The key of a song is the tonal center of the music. In other words, it is the chord which a piece of music revolves around. It can also be called the “I chord”, in roman numerals, or the tonic chord.
For instance, if you’re told that the song you’re about to learn is played in the key of C Major, that means that the main notes that will form the melody and the chords of that song belong to the C Major scale.
This doesn’t mean that there can’t be other notes of chords, but the main structure and elements will be defined by the key of the song.
In short, the tonic is what feels as if you’ve landed “home”, you feel the release of tension and a sense of stability when you come back to the I chord.
How to Quickly Figure out a Song’s Key?
Many songs start and/or end with this “I chord”, or the tonic. This isn’t a rule of any kind, although it is true in the majority of cases.
With this in mind, you can listen for the chord in which a song ends, and many times, that chord is the key of the song.
Many songs also start on the IV degree, and they might end on a chord that isn’t the tonic, and there’s nothing wrong with that.
A couple of very easy examples that allow you to hear this clearly are the following songs:
- Givin the Dog a Bone, by AC/DC, is a song in A Major, and it ends on that chord, giving you a clear sense of resolution.
- Let Me Put My Love Into You, also by AC/DC, is a song written in E minor, but if you listen closely, you’ll notice that the song ends in A. If you listen to the whole song and interiorize that low E as your “home” or “ground”, it will be clear that the final chord leaves some kind of open space, lacking a full resolution.
Songs like the second example are the reason why this method does not work 100% of the time, but if you train your ear enough, you’ll easily realize when the ending chord does not correspond to the key of the song.
The key signature can let you know in which key a piece of music has been written in very easily, you only need to understand a couple of basic concepts.
It is comprised of a number of sharps or flats that are placed just before the time signature of the composition. These must be placed in a specific order, and they are what allows you to transform a tune in C Major (no sharps or flats) into something else such as G Major (1 sharp).
The order in which these accidentals must be placed follows the circle of fifths, which you can see in the image below:
Here’s how this works:
- You start in C Major, and start moving clockwise in fifths (C to G, G to D, D to A, etc);
- Each time you move clockwise, you add a sharp according to the following order: F, C, G, D, A, E, B;
- If you move counter-clockwise, you will be adding flats instead of sharps, in order to get the flat keys such as Eb or Ab;
- You add flats in the opposite order of sharps: B, E, A, D, G, C, F;
- For each major key, there is a relative minor key which has the same accidentals. Every major key has its relative minor a major sixth above it (for example, G Major and E minor are relative keys, meaning they share the same accidentals).
You should have your own copy of the circle of fifths until you have interiorized it, it is useful for lots of exercises too.
Now that you know how it works, if you notice that the sheet music for a certain tune has only one sharp in its key signature (as shown below), you immediately know that it will either be in G Major or in E minor (its relative minor key).
Determining the Key of a Song by Analyzing its Chords
If you’re learning a song by ear, you won’t have the sheet music telling you in which key it has been written. Maybe you already have a chord chart available, and this is also a valid way to figure out the key, as long as you understand the basics of harmony.
If you know how to tell which chords fit into a certain key, by looking at the chords of a certain song, you can easily deduce the key of the song. Below, you’ll find the chord chart for C Major, with the chord degrees and the corresponding chords.
There is something that you absolutely must know by heart. In a Major key:
- The degrees I, IV and V are always Major;
- The degrees II, III and VI are always minor (which is why they’re sometimes written ii, iii and vi);
- The degree VII is always diminished;
This is valid for triads, if you do the same with seventh chords, there will be some slight differences.
And why is this useful, you might ask?
Let’s imagine you find the chord chart to the song you want to learn, but you haven’t been able to figure out the key just by listening to it and trying to identify that “I chord”.
If you notice that the chords D minor and E minor are present, that would be a potential indicator that you’re in C Major, as those would be the II and III chords.
Identifying F and G Major is also a way to realize we’re likely in C Major, as those would be the IV and V chords. If we were in G Major, for instance, F would have to be F diminished and not Major.
If the song is in a minor key, the chord chart is the same, but it would begin on the 6th degree of its relative major key.
For example, for C Major’s relative key (A minor), the chords are the following:
A minor – B diminished – C Major – D minor – E minor – F Major – G Major
Final Thoughts on Finding the Key of a Song
As you can see, knowing a lot of music theory isn’t an essential part of being able to figure out the key of a song, although it might help in some cases, when you don’t have all of the resources that could better inform you.
The most important thing is that you keep training your ears whenever you can, and at a certain point, a task like this will become second nature to you.