The tonic note in C Major is C, for instance. This is especially important if there are no flats or sharps in the notation. Purely natural notation will either be keyed in C Major or A Minor. Look for patterns building around one of these two notes. [2] X Research source

Chord maps outline the chords of each key clearly. [3] X Research source For example, the chords in the key of C are: C Major, D Minor, E Minor, F Major, G Major, and A Minor.

For flats, do the opposite with E, located at the bottom of the Circle. Count counter-clockwise from E based on how many flats are in your key signature. The Circle of Fifths is designed for Major keys. A Minor key will use the same accidentals, except the tonic note will be three steps down from the Major.

If the final note is a chord, look for the “root note”, or lowest in the chord. This technique may not work if the music you’re trying to decipher is experimental or avant-garde.

Major keys are so-called because they sound “happy”. Minor keys sound naturally sad in contrast.

Major keys are typically purer than minor. If there is a mixture of happy and sad-sounding chords in a piece, chances are high it is a minor key.

The Beatles’ classic “Yesterday” is an interesting example due to its sophisticated chord progression. Play around by humming a few different notes, and try to hum the note as long as possible. If you find yourself stumped, hum the F note. Although certain notes may fit better than others, Yesterday is in the key of F Major. [6] X Research source You may have to try out a few different notes before you hear one that sounds synchronous with the music.

Play along to The Beatles’ Yesterday using one note. If you play F, you’ll find the note backs up the chords used in the song. There are tuner apps online and on your phone that will tell you what note it is you’re singing.

Sites like Key Finder are specifically around for people to research the key signatures of famous songs.

For major-keyed songs, The Beatles’ Yesterday and the Village Peoples’ YMCA are great exercises. Yesterday in particular is a good exercise because of how deceptively tricky it is. For minor-keyed songs, try Nirvana’s Smells like Teen Spirit and Michael Jackson’s Beat It. Both of these songs have been researched extensively already, and it won’t be difficult to find info on these pieces if you need it.

Song bridges make something of an exception. It’s a common pop songwriting convention to bring a section of a song into a different key. This convention is used to refresh the song towards the end.

Modern pop and rock songs don’t tend to have sophisticated notation. You may only find chord progressions and basic melodic notation for pop songs. Even so, this information should give you enough information to deduce the proper key. [8] X Research source