Why does vi's y yank two lines?

Question

nyy yanks n lines, which is normal behaviour, but why does y yank two lines while yy yanks one line only?

It sounds rather counterintuitive to me. I wonder what led to this decision. Not sure about vim since I'm using vi for now.

Answer 1

Single y has to be followed by an operand. Probably what you are doing is pressing 'y' then one of the movement commands 'j' or 'k'. This command also acts as an operand for 'y' so it yanks to lines. Tray yj and yk to see which line gets yanked.