In the first diagram, the letters indicate plausible opening moves by Black, excluding symmetrically equivalent moves. These can be anywhere apart from the first and second line.

The remaining diagrams show for each plausible opening move by Black the usual answers by White. They are not Black’s second moves, which of course depend on what White does.

Note that in go it is normal to number individual moves rather than (as in Chess) pairs of moves; hence the “second move” of the game is White’s first move.