Major 6th Chord

The major 6th chord -- formed by adding a major 6th above the root.
The note added on to a C major triad is A.

In fake chart, it is indicated by the number 6.

1-3-5-6


Some charts will call it added 6th.
Or add6.

The major 6th chord -- add some character to the pure sound of a major triad -- not really dissonant to our ears.
It is so smooth sounding that it can come across as a bit old fashioned if used extensively.

It is frequently acceptable within certain musical styles -- to add a 6th to major triads even if the lead sheet doesn't so indicate.

Most often, the 6th would be done to I & IV without introducing any accidentals (sharps or flats)

But not on the V.

This is cuz the V chord is to bring us back to the I chord.
Our ear tells us it must eventually resolve to the I chord.

THe addition of th 6th lessens the V sense of direction and a strong V-I progression would never occur.