Overlapping Strings

We can find the exact same note (or unison) by going back 5 frets towards the headstock on the string immediately below as we look down across the strings towards the floor . This shift can be applied from any note at any fret. 

The exception though is moving a note from string 3(G) to 2(B). To do so, you must go back 4 frets towards the headstock.