Byrne Variation

Pirc Defense

1.e4 d6 2.d4 Nf6 3.Nc3 g6 4.Bg5 c6

White pins the knight with Bg5, and Black answers with the flexible ...c6.

The idea

After 4.Bg5 c6, White's bishop pins the knight on f6 and can support Qd2 and Bh6 later. Robert Byrne introduced 4.Bg5 in the 1960s. Black often plays 4...Bg7, but Wikipedia calls 4...c6 more flexible, saving a tempo against Qd2 and Bh6.

Plans for White

Develop with Qd2 and f3 or Nf3, and decide between castling queenside with a kingside attack and a quieter setup. The bishop on g5 makes Bh6 easy once the queen is on d2.

Plans for Black

Gain space on the queenside with ...b5, and develop with ...Nbd7 before committing the bishop. Keeping it on f8 for now is the tempo Wikipedia mentions: if White plays Qd2 and Bh6, Black's bishop hasn't spent a move on g7 only to be traded.

Watch out for

For Black, the pin. With the knight on f6 pinned and White ready for e5, Black must watch for the knight being driven away or for Bxf6 damaging the pawns.

For White, the queenside. Black's ...b5 and ...b4 can hit the knight on c3 and the e4 pawn behind it.