Finding the key of a song can have many applications. Composers can use it to transpose and modify the scale to better suit a singer’s voice or make the arrangement sound better. DJs can use it to make sets which flow better. To find the key, all that is required is a very basic understanding of music theory and a musical instrument, preferably a piano.
First, listen to the song a number of times and hum along with the main melody. After a few listens, focus on the main note implied by the melody. Force your brain into picking just one note and try humming that one note continuously. Find that note on a piano by matching it with your humming. Keep trying different notes until it matches.
There’s a high probability that this note is the root note (also known as tonic) for that song. The other times, it usually ends up being the 3rd or 5th in the scale. If the root note is correct, it will fit almost everywhere in the song.
Once the root note is set, go up by either 3 or 4 semitones and check for the one that “sounds right”. Depending on which one fits, the scale is either minor or major. We can build the rest of the scale by using a simple guide.
Typically, major and minor scales are built using 7 notes. The 8th note will be the root again in the next octave. Almost every note in the scale (except the root and the 5th) will have major and minor versions, depending on the scale. Every scale starts with the root note. After that, we go up the scale using the formula below:
R = Root note
S = Semitone
T = Tone (two semitones)
For example, take the root note to be C. By using the above formula, the C major scale would be:
Similarly, the C minor scale would be:
Sometimes, a few notes might be off depending on the song. For getting them right, play around with whatever note sounds off by shifting it up or down by a semitone. After making these changes, check whether the melody of the song is built using only these notes. If so, then that is the scale being used for that song.
Similarly, there are several other scales which need not have 7 notes. For example, pentatonic scales have 5 notes. A chromatic scale has 12 notes. There are different formulae for building these scales.
It is important to note is the relationship between the major chords and relative minor chords, and how they are linked with the 6th in the scale.
For example, the A minor chord is the relative minor of C major. In the C major scale, A is the 6th. This applies to various other scales. Suppose we are in the scale of G major. The 6th is an E, so E minor is the relative minor to G major.
This approach can help you shift from one chord to another after finding the tonic of the song. In many popular songs, chord progressions are based around I-V-vi-IV, and thus, relative minors are used as part of the chord progressions. Understanding the circle of fifths helps a lot in this regard. Some songs also modulate (i.e change key) within the song. In such cases the song may change its tonal centre during the course of the song.