Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...Nf6/4. O-O/4...Ng4

= Ruy Lopez: Berlin Defence, Fishing Pole Variation =

4...Ng4
With 4...Ng4, Black enters the Fishing Pole Variation. Here, the logical move is 5.h3 to attack the knight. Black can then set up a trap by playing 5...h5, creating the so-called fishing pole of the Fishing Pole Variation: the h3-pawn is the fish, the g4-knight is the bait, and the h5-pawn is the hook. Black is trying to tempt White to play 6.hxg4; White can sidestep the trap by simply continuing to develop normally, allowing them a comfortable advantage.

If White falls for the trap by playing 6.hxg4, then 6...hxg4, opening the h-file and attacking the f3-knight. Here, the best plan for White is to allow the knight to be captured. White should play either 7.c3 or 7.Bxc6, disallowing Black's knight from moving to d4 to attack f3. Black has an advantage due to the weakness of White's kingside.

If 7.Nh2 instead, intending to save the knight, then 7...Qh4 is crushing. 7.Ne1 loses even faster.

Theory table
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 * 1.e4 e5 2.Nf3 Nc6 3.Bb5 Nf6 4.O-O Ng4