4.Qc2 Qh4

English Defense

1.d4 b6 2.c4 e6 3.e4 Bb7 4.Qc2 Qh4 5.Nd2 Bb4

White protects e4 with the queen, and Black's queen comes out to hit it at once.

The idea

After 4.Qc2 Qh4 5.Nd2 Bb4, Black's queen and bishop are both active early, hitting e4 and pinning the knight on d2. This is how Viktor Korchnoi beat Lev Polugaevsky in game 6 of their 1977 Candidates semi-final in Évian.

Plans for White

Develop with Ngf3 to hit the queen, Bd3 and castling, and keep e4 protected. White's pieces are a little tangled, so the priority is to untangle them before Black's quick development causes trouble.

Plans for Black

Develop with ...Nf6, ...Nc6 and ...O-O, and keep pressure on e4 and d4. Black's queen on h4 is active but can be chased by Ngf3, so be ready to move it to a useful square.

Watch out for

For Black, the queen. Coming out early is the point of the line, but each attack on it gains White time, so make sure it has safe squares.

For White, the pin on d2. Until White castles or moves the bishop, the knight on d2 is pinned and e4 has one fewer defender.