In GuitarOS: Practical Theory, we walk you through:
- how to ID notes on your fretboard by their actual names,
- which notes go into which keys,
- how to build simple & fancy chords,
- which chords belong in each key,
- what the “function” of each of those chords is,
- how chords are combined into progressions,
- how to speak the language of rhythm, and
- how all of this gets written down in a way that’s useful to you AND the musicians you collaborate with.
But I get it—today you’re in a hurry and you want to know:
How do I figure out the key of a song?
Here’s a list of questions that will help you decide.
Although putting this in a checklist makes it seem really formal, this is something that will eventually happen automatically, in a matter of seconds.
I trained my brain to do this, and you can too.
With a little practice you’ll be able to take a snap “reading” of a song (or a section of a song).
I’m going to explain each of these in order, but I want to begin by saying that you should be going through this checklist in this order when trying to determine a song’s key.
Probably the easiest way to determine what key you’re in is when the written music tells you the key.
On a chart written by a literate musician, there will be key signature—some number of sharps or flats between the clef and the time signature:
In this example, one flat means that the song is in F.
Of course, if someone says, “this is a blues in G” or “we’re playing Dock of the Bay, but in F,” you’ll obviously know what to do.
If you’re learning a song from a recording, or reading a chart written by a less-than-literate musician, you can look at the first & last chords of the song—they’re frequently the I chord.
It’s not a hard and fast rule though. Some common exceptions:
- LOTS of songs don’t start on the I chord.
- Some never go to the I chord.
- There are plenty of songs that purposely end on a chord other than the I (which gives the ending an unsettled feeling).
- And of course if the song changes to a new key somewhere along the way, that’s going to complicate matters.
If we look at the extended chords that show up diatonically (diatonic = made up of only notes from within the key), we see that there’s only one that’s a dominant 7 chord: the V.
If you see a set of chords that only has one dominant 7 among them, there’s a pretty good chance that that chord is the V.
You can also be on the lookout for other mini-progressions like ii-V or IV-iv.
When I see Am-D, my brain goes straight to “ii-V in G.”
When I see F-Fm, my brain goes right to “IV-iv in C.”
(Again, “diatonic” = only using notes from within the key—no outside notes.)
If you see this progression:
…you can see that the only key that has all of those chords diatonically is G:
So even though there’s no G chord, it’s in the key of G.
(This example is from Dave Matthews Band’s #41. The band eventually does go to the G chord, but it takes them a minute and a half of this progression before they do.)
7. Are most of the chords diatonic to a key, with some non-diatonic (but not entirely unexpected) chords sprinkled in?
Here are the chords from John Mayer’s Badge & Gun:
All of the chords are diatonic to G… except that A7 in measure 6.
But when we look at the commonly-occurring non-diatonic chords, we see that A7 isn’t entirely unexpected:
Sweet Home Alabama goes D, Cadd9, G.
Is that I-bVII-IV in D? (in red below)
Or a V-IV-I in G? (in purple below)
If you do a google image search for “Sweet Home Alabama sheet music,” you’ll see that roughly half of the transcriptions show it in D, and half in G.
My personal preference is to think of Sweet Home Alabama in G, because it’s easier that way.
But thinking of it in D is just as correct.
For some tunes, it doesn’t really make sense to think of them in one single key.
Here’s The Allman Brothers’ Melissa:
Although we don’t leave the key long enough to warrant writing in a key change, for measures 11 & 12 we’re very clearly playing I-ii-iii-IV in the key of A.
If you were soloing over these changes, the E major scale that sounded so good over the rest of it suddenly doesn’t fit quite right—you have to smoothly switch to A major for those two bars.
Jazz music is filled to the brim with this sort of thing—there’s ii-V-I, and then I becomes minor and it’s the ii of some other key…
Jazz musicians still use this same roman numeral language, but they’re not beholden to whatever key is written on the chart—they’re talking about what key the song is in for this subsection of the tune.
Playing fluidly over chords that change key frequently requires an equally fluid mindset about keys.
Learning to see subsections of songs as being in their own key is the logical extension of getting good at seeing songs as being in a key.
We’re going to touch on this again next week when we talk about how all of this feeds into our understanding of modes.
What About Minor Keys?
It’s worth mentioning that up until now we have studiously avoided talking about minor keys—keys where the “one chord” is minor: i.
Minor keys are their own thing, and worthy of their own article (which we’ll get into in two weeks, after our discussion of modes).
- At the top of a well-written chart, you’ll see a clef & a time signature, and in between them is a key signature—the number of sharps or flats tell you what key the song is in.
- If the last chord in the song gives you a sense of resolution, it’s probably the I.
- The only diatonically occurring dominant chord is the V. If you see a dominant chord, there’s a decent chance that it’s the V.
- Other clear chordal movement that suggest a key are ii-V and IV-iv.
- Sometimes you’ll be able to see that all of the chords are diatonic to a certain key.
- Even if the I isn’t one of those chords.
- Other times most of the chords will be diatonic to a key, and the non-diatonic chords will be common enough.
- Some songs be written in more than one key.
- One is usually easier than the other.
- Some songs move fluidly through different keys.
- Getting good at quickly seeing the key of simpler songs will lead to getting good at spotting those times when a subsection of a song is in a different key.
- Want this chord chart as a PDF & in Google Sheets? You can snag that here.
- if you’d like to be walked through this one piece at a time, sign up for GuitarOS