When doing diatonic triads on the neck, they always follow this pattern in the same position. The 2-1, 1-2, 1-1-1 trick is a triad fingering pattern indicating that 2-1 two notes on a string and then one note on the adjacent string. 1-2 with one note on a string then two notes on the adjacent string. Finally 1-1-1 with one note on every string.
So, say you start on fret 5 for an A major triad: (1-2)
A on Fret 5, string 6, C# on fret 4 string 5 and E on fret 7 string 5. So, that pattern is the 1-2. A is the only note on the 6th string and C# and E are two notes on the 5th string.
Then in the pattern is the 1-1-1:
For the Bm triad is B Fret 7 on the 6th string, D on the 5th fret 5th string, and F# 4th fret on the 4th string. One note per string hence the pattern 1-1-1.
Finally the last cycle is 2-1 for the C#m triad
C# is 4th fret 5th string, E is 7th fret 5th string and G# is 6th fret 4th string. Two notes on the 5th string and one on the Forth string. the 2-1.
Now the pattern starts over for the D major triad on the 5th string.
See if you can try the pattern yourself the rest of the way up:
A
Bm
C#
D
E
F#m
G#dim
A
