Italian Game:Two Knights Defense, Polerio Defense

The main answer to 4. Ng5, named after Giulio Cesare Polerio. Black gives back the pawn with 5...Na5, chases the bishop and gets quick development for it.

It is reached after 10 moves: 1. e4 e5 2. Nf3 Nc6 3. Bc4 Nf6 4. Ng5 d5 5. exd5 Na5. It is a line of the Italian Game.

White is to move and has 37 legal moves. Over the next four moves — two for each side — the game can branch into 1,734,534 different games. Those games reach 588,187 different positions: most positions can be reached in more than one way, 2.9 on average.

The Lichess opening list names 13 further lines after this position, such as Bishop Check Line (6. Bb5+) and Kieseritzky Variation (6. d3).

The same moves can be played in 495 different legal orders that all reach this position. For example 1. Nf3 e5 2. e4 Nc6 3. Bc4 Nf6 4. Ng5 d5 5. exd5 Na5, which starts as the Zukertort Opening.