Picking the right chords is a little intuitive but I'll try to explain this as simple as I can. Here's the most basic brief idea:
Lets start with 3 major chords for a simple song "Twinkle, twinkle little star" in the key of C... We'll try to pick the right chord for each note of the song.
The notes for the first line should be something like:
|| C C G G | A A G , || = Twin-kle Twin-kle lit-tle star
(here each letter is 1 note played 1 beat long. Comma indicates hold/sustain the previous note)
The 3 basic chords in key of C is C major, F major, G major. Breaking them up into their individual notes, they are:
C maj = C + E + G
F maj = F + A + C
G maj = G + B + D
Now we pick the chords so that the chord contains the note played. This gives us:
|| C , , , | F , C , ||
You might wonder why not use F maj when C note is played or G maj when G note is played. This is for 2 reasons:
1. Try to minimise chord changes by picking a chord that covers as many of the notes in the bar as possible
2. In the key of C, you want to make the song return to the C chord as much as possible to put stress on the root chord.
You will get better at identifying which chord needs stress with more practice. Let me know if this made any sense at all...
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