2 notes is not calles a chord yet , it starts when you use 4 or more notes , thats how it is on the guitar and piano . Like , metal rhythm guitarist also play A,B,C,etc , but only with 6and 5 string or with 4 and 3 , they use minor chords to make it cool .
Also Lazors , it is theory , Why do i say A,B,c ,etc ?
C=do
D=re
E=mi
F=fa
G=sol
A=la
B=si
This are the basics my friend , if you don't know this you can't play . If somebody asks you to play the song in Do or in sol , you need to know those scales .