user16771: I would like to know why playing Bf5, allowing White to exchange the white-squared bishops, doubling Black's pawns is good for Black.
In such situation, castling on either side seems quite unsafe for Black given its weaknesses of b6, h6 and probably f6 later on.
Plus, I am not sure eliminating the White bishop is a good reason, as it is not particularly dangerous in such a position.


user19877: First, it is just one way to play it. It is not really better or worse than other plans at this juncture.
Really, the big problem is that I am not a fan of 7...g6 since what is the Bf8 really going to do on g7? It is biting on granite, and is better placed on e7 defending the k-side, and eyeing the white q-side, or even Bd6 eyeing a trade for the active Bf4. Bf4 is technically a bad bishop, but it is already outside the pawn chain, and active, so it is OK to trade it off. 
The Bg7 would gain activity if you could ever get in e5, but I do not see that happening. After just a few more normal developing moves (Bg7; Nf3) this already scores at about 70% for white in the Mega 2019 database, so even practically speaking, g6 and Bg7 is not a plan I would want to defend with.
Brian is right, and I have mentioned this before, about trading off a bishop when you can place all the remaining pawns on the opposite color of the traded bishop. If you trade off the light-squared bishop, being able to place all the pawns on the light squares keeps control of them is one favorable aspect of the position. I disagree with him to the extent that this "very much favors black", otherwise we would see this a lot more often.
It is just one plan that, in reality, has pluses and minuses like most plans. This is very common in the slav lines where black plays Bg4xNf3 with the pawns on c6-d5-e6. It unbalances the position, so if you need a win, this is one thing to consider as a plus; but you have to keep in mind that you did cede the bishop pair, so you have to be incredibly careful about allowing the position to open up later. In practice, the side with the bishop pair almost always is able to open the position eventually, often in the endgame, so you have to make thing happen, or keep everything under tight control to prevent that. Often it is ideal to try and trade the other bishop for its equal counterpart, at least destroying the opponent's bishop pair.

user4418: After the exchange of white squared bishops black will play e6 cementing the center, leaving all his pawns on white squares. The white squared bishop is Black's bad piece in these structures and conversely it is white's best piece. The exchange very much favours black.
With black pawns on d5 and f5 white isn't going to be opening the center any time soon so the black king is safe in the center. Black has at least equalised.

16771: <p>I would like to know why playing Bf5, allowing White to exchange the white-squared bishops, doubling Black's pawns is good for Black.</p>

<p>In such situation, castling on either side seems quite unsafe for Black given its weaknesses of b6, h6 and probably f6 later on.</p>

<p>Plus, I am not sure eliminating the White bishop is a good reason, as it is not particularly dangerous in such a position.</p>

<p><a href="https://i.stack.imgur.com/SEw7M.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/SEw7M.jpg" alt="l"></a></p>
 19877: <p>First, it is just one way to play it. It is not really better or worse than other plans at this juncture.</p>

<p>Really, the big problem is that I am not a fan of 7...g6 since what is the Bf8 really going to do on g7? It is biting on granite, and is better placed on e7 defending the k-side, and eyeing the white q-side, or even Bd6 eyeing a trade for the active Bf4. Bf4 is technically a bad bishop, but it is already outside the pawn chain, and active, so it is OK to trade it off. </p>

<p>The Bg7 would gain activity if you could ever get in e5, but I do not see that happening. After just a few more normal developing moves (Bg7; Nf3) this already scores at about 70% for white in the Mega 2019 database, so even practically speaking, g6 and Bg7 is not a plan I would want to defend with.</p>

<p>Brian is right, and I have mentioned this before, about trading off a bishop when you can place all the remaining pawns on the opposite color of the traded bishop. If you trade off the light-squared bishop, being able to place all the pawns on the light squares keeps control of them is one favorable aspect of the position. I disagree with him to the extent that this "very much favors black", otherwise we would see this a lot more often.</p>

<p>It is just one plan that, in reality, has pluses and minuses like most plans. This is very common in the slav lines where black plays Bg4xNf3 with the pawns on c6-d5-e6. It unbalances the position, so if you need a win, this is one thing to consider as a plus; but you have to keep in mind that you did cede the bishop pair, so you have to be incredibly careful about allowing the position to open up later. In practice, the side with the bishop pair almost always is able to open the position eventually, often in the endgame, so you have to make thing happen, or keep everything under tight control to prevent that. Often it is ideal to try and trade the other bishop for its equal counterpart, at least destroying the opponent's bishop pair.</p>
 4418: <p>After the exchange of white squared bishops black will play e6 cementing the center, leaving all his pawns on white squares. The white squared bishop is Black's bad piece in these structures and conversely it is white's best piece. The exchange very much favours black.</p>

<p>With black pawns on d5 and f5 white isn't going to be opening the center any time soon so the black king is safe in the center. Black has at least equalised.</p>
