What are the reasons why I should not try to take a Knight with my Bishop if it'll result in doubled pawns?

Question

For instance, in the Ruy Lopez, after

1. e4   e5
2. Nf3  Nc6
3. Bb5  a6

why shouldn't I take the Knight?

4 Bxc6 dxc6

Now Black has a doubled pawn.

It isn't a very popular line of play, which is why I think I shouldn't play it; but as to the principle, is it because I'm opening up Black's game? I don't see any significant advantages/disadvantages for either side.

Answer 1

This is a variation that was played fairly frequently, but it has been more or less worked out to a draw, so it is less common at the top levels now.

The Ruy Lopez is a very concrete opening, and the capture on c6 has to be carefully considered as it relates to the exact position.

In terms of opening principles, you're giving up a bishop for a knight in an open position. If you could take the pawn on e5, it might be good, but the pawn is protected via the tactical shot 5.Nxe5? Qd4! when white has basically given up the two bishops for very little compensation in a wide open position.

Another downside to Bxc6 is that the black's light square bishop is developed by dxc6, allowing it to come to g4 without waste of time. Indeed, one of the most critical lines continues 5. O-O Bg4 6. h3 h5 when black is offering a piece for attacking chances along the h file.

With all of that said, this is a very common motif in the Ruy Lopez (Bxc6). The pawn structure after d4 and exd4 is very good for white. If all of the pieces were traded off, white would win the king and pawn endgame by making a passed pawn on the kingside and black cannot make a passed pawn on the queenside.

There are also many cases when spoiling your opponent's pawn structure is very good. For example, if black has castled kingside, and a bishop can take on f6 and force ...gxf6, this is almost always good for white. The king becomes very exposed with the g file open. In the Spanish Four Knights, one player or the other will very frequently take on c6 or c3.

Answer 2

• Some impressive people consider bishops marginally stronger than knights, though the board position is critical. Reference below.
• The black Q is freed, perhaps saving black a tempo.
• The black w-squared B is freed, saving a tempo.
• The white w-squared B is gone, after 2 moves were spent on it.
• Black's pawns are doubled, but they still hit the d-file. Not so bad.

In the next 5 moves, it is possible for black to develop all 3 remaining minor pieces, the queen, and castle. In short, black has a flexible position.

Edit

Some people have issues with the statement about bishops being stronger than knights. So check this Wiki page (OMG Wiki). You'll see a chart down under 'Alternate Valuations.' You'll see references that include names of knowledgable people that rate the bishop higher. No one listed on the chart rates knight higher, though many rate them equal.

Answer 3

If the doubled pawns created after taking the Knight leave a b- or g-file half open against your castled King, it might be relatively easy to create an attack against your King if the opponent's Rook leverages the half open b- or g-file by building pressure against your King's castle.