1. Determine the key

First of all, you must distinguish whether a song is in a major key or a minor key. Generally, songs in major keys usually start with one of the notes of 135 and end with one of the notes of 15; songs in minor keys start with 36 and end with 6. So if a song ends on tonic 1 it's probably in a major key, and if it ends on tonic 6 it's probably in a minor key.

But if the end of the song is neither 1 nor 6, it is necessary to analyze the tonal tendency of the song melody. If the 135 notes often appear in the melody, it can be determined to be a major key; if the 613 notes often appear in the melody, it can be determined to be a minor key.

2. Initial chord matching

If the chords in the key you are more familiar with often appear 135, you can conclude that it is a major key; that is, C in the major key and Am in the minor key often appear 135 These sounds can be judged to be major keys; the chords are first assigned to the song, and then converted to the actual key through the relationship between the keys.

1. First add chords to the beginning and end of the song. The major key is C and the minor key is Am.

2. The section before the end is usually accompanied by a dominant chord or a dominant seventh chord. The major key is G or G7, the minor key is Em or E7.

3. If appropriate, it should be preceded by a subordinate chord before the dominant seventh chord. The major key is F and the minor key is Dm.

4. Match a measure that is obviously inclined to a certain chord with that chord.

(1) The sounds in the section are all internal tones of a certain chord.

(2) The note on the downbeat in the measure determines the chord.

(3) The longer note in the measure determines the chord.

(4) The recurring notes in the bar determine the chord.

(5) The decorated note in the bar determines the chord.

3. Repair the chords again

Based on the initially arranged chords, it is necessary to use the functions of the chords at all levels of the scale to find proxy chords to beautify the chords again.

1. The first-level chord, the main chord C, is the only decisive and controlling chord, so it is often used at the beginning and end.

2. The secondary chords Dm, D7, and Dm7 have a timbre close to the G7 chord, so they can be used as the preceding chord of G7 to make the chord progression smoother.

3. The third-level chords E and Em are weaker chords. They are usually used as substitute chords for the first-level chords when the first-level chords occupy too many bars.

4. The fourth-level chord F is based on the fourth tone of the scale. It makes the chord tone richly varied and is used to connect the main chord and the dominant chord.

5. The fifth-level chords G and G7 have the property of strongly moving towards the main chord, so they are also called leading chords.

6. The timbre of the sixth-level chord Am is between the first and fourth levels, and can be used to represent these two chords or to connect the two.

Extended information:

According to the relationship of third intervals or non-third intervals, the combination of three or more tones is called a "chord". Chords composed according to the relationship of third intervals are widely used because they maintain a certain degree of tension between each tone, the sound is harmonious and full, and it conforms to the natural laws of overtones.

Popular music generally only talks about major chords, and rarely uses minor as a mode to redefine chords, because the phenomenon of using related major and minor chords in pop music is very common. If It would be confusing to explain it using classical harmony.

There are 7 basic chords: C, Dm, Em, F, G, Am, Bdim, which are 1st, 2nd, 3rd, 4th, 5th, 6th and 7th level chords respectively. 1, 4, 5 are called positive triads, and 2, 3, 6 are called minor triads. 7th degree chords are extremely rarely used in popular music. Level 1 chords are also called tonic chords, level 4 chords are also called subordinate chords, and level 5 chords are also called dominant chords.

Reference: Baidu Encyclopedia-Chords