user-1: [title "Portisch-Radulov, Moscow 1977"]
[fen "2r2rk1/pb1n1ppp/1ppqp3/8/2PPQ3/1P3NP1/P4PBP/R2R2K1 w KQkq - 0 1"]

Doesn't 16.c5 just lose a pawn?

user19877: The short answer is “no” due to the tactics after 16. c5 bxc5 17. dxc5 Nxc5? 18. Qb4 Qe7 19. Rdc1 winning the piece. This is a famous combination from some books I have read.
The position is really a lot deeper than just that tactic, and it is worth mentioning that great move, and why Portisch played it.
First, even though not all of this came to fruition, Portisch probably thought about a lot of this. c5 did the following things:
It deadened the Bb7 on the more important h1-a8 diagonal. It might still have found a home on the f1-a6 diagonal, but there it would not be anywhere near as effective on that diagonal since there was nothing to attack there. It made it a partially bad bishop. Later, on move 25, Portisch also made the great decision to trade on d5, leaving a good N vs. bad B. That affected the game to the end when he finished with a pretty combination.
Although black will get control of d5 with Nf6-d5, with tempo even due to the Qe4, the d6 square looks like a very nice spot for the Nf3-d2-c4-d6. It might take four moves to get there, but it would have been very powerful on that square. It turns out that how the game played out, Portisch preferred to put the Nf3 on e5, where it still was intolerable, and kept black tied to c6. Note that, as Fischer said, you have to give squares to get squares; but I will add that if they are for knight outposts, usually it is a good trade if your knight outpost is much deeper in the opponent’s territory that his is in yours. So, giving up d5, which is not even on your side of the board, in exchange for an outpost on e5 or d6, it is a good trade in most cases. It can really divide the board for your opponent when it is posted that deeply on the other side of the board. As it turned out, black felt forced to weaken his king-side and dark squares further with f6 because the Ne5 was so well-placed.
 [Event "EU Team-ch06 Final"]
 [Site "Moscow"]
 [Date "1977.04.??"]
 [Round "6.1"]
 [White "Portisch, Lajos"]
 [Black "Radulov, Ivan"]
 [Result "1-0"]
 [ECO "E08"]
 [WhiteElo "2625"]
 [BlackElo "2500"]
 [PlyCount "79"]
 [EventDate "1977.04.13"]
 [EventType "team-tourn"]
 [EventRounds "7"]
 [EventCountry "URS"]
 [FEN ""]

 1. Nf3 Nf6 2. g3 e6 3. Bg2 Be7 4. O-O O-O 5. c4 d5 6. d4 c6 7. Qc2 b6 8. Rd1 Nbd7 9. b3 Bb7 10. Nc3 Rc8 11. e4 dxe4 12. Nxe4 Nxe4 13. Qxe4 Qc7 14. Bf4 Bd6 15. Bxd6 Qxd6 16. c5 Qe7 (16... bxc5? 17. dxc5 Nxc5? {This makes it worse} 18. Qb4 Qe7 19. Rdc1 $18) 17. b4 Rfd8 18. Qc2 Nf6 19.Ne5 Nd5 20. a3 b5 21. Be4 g6 22. h4 a5 23. bxa5 Ra8 24. a4 b4 25. Bxd5 Rxd5 26. Rab1 f6 27. Ng4 Rad8 28. Rxb4 Bc8 29. Ne3 R5d7 30. Rb8 e5 31. d5 cxd5 32. c6 Rd6 33. c7 Rf8 34. Rxd5 f5 35. Qc5 Re6 36. Qxe7 Rxe7 37. Rxc8 Rxc8 38. Rd8+ Re8 39. Rxe8+ Rxe8 40. Nd5 1-0


-1: <pre><code>[title "Portisch-Radulov, Moscow 1977"]
[fen "2r2rk1/pb1n1ppp/1ppqp3/8/2PPQ3/1P3NP1/P4PBP/R2R2K1 w KQkq - 0 1"]
</code></pre>

<p>Doesn't 16.c5 just lose a pawn?</p>
 19877: <p>The short answer is “no” due to the tactics after 16. c5 bxc5 17. dxc5 Nxc5? 18. Qb4 Qe7 19. Rdc1 winning the piece. This is a famous combination from some books I have read.</p>

<p>The position is really a lot deeper than just that tactic, and it is worth mentioning that great move, and why Portisch played it.</p>

<p>First, even though not all of this came to fruition, Portisch probably thought about a lot of this. c5 did the following things:</p>

<p>It deadened the Bb7 on the more important h1-a8 diagonal. It might still have found a home on the f1-a6 diagonal, but there it would not be anywhere near as effective on that diagonal since there was nothing to attack there. It made it a partially bad bishop. Later, on move 25, Portisch also made the great decision to trade on d5, leaving a good N vs. bad B. That affected the game to the end when he finished with a pretty combination.</p>

<p>Although black will get control of d5 with Nf6-d5, with tempo even due to the Qe4, the d6 square looks like a very nice spot for the Nf3-d2-c4-d6. It might take four moves to get there, but it would have been very powerful on that square. It turns out that how the game played out, Portisch preferred to put the Nf3 on e5, where it still was intolerable, and kept black tied to c6. Note that, as Fischer said, you have to give squares to get squares; but I will add that if they are for knight outposts, usually it is a good trade if your knight outpost is much deeper in the opponent’s territory that his is in yours. So, giving up d5, which is not even on your side of the board, in exchange for an outpost on e5 or d6, it is a good trade in most cases. It can really divide the board for your opponent when it is posted that deeply on the other side of the board. As it turned out, black felt forced to weaken his king-side and dark squares further with f6 because the Ne5 was so well-placed.</p>

<pre><code> [Event "EU Team-ch06 Final"]
 [Site "Moscow"]
 [Date "1977.04.??"]
 [Round "6.1"]
 [White "Portisch, Lajos"]
 [Black "Radulov, Ivan"]
 [Result "1-0"]
 [ECO "E08"]
 [WhiteElo "2625"]
 [BlackElo "2500"]
 [PlyCount "79"]
 [EventDate "1977.04.13"]
 [EventType "team-tourn"]
 [EventRounds "7"]
 [EventCountry "URS"]
 [FEN ""]

 1. Nf3 Nf6 2. g3 e6 3. Bg2 Be7 4. O-O O-O 5. c4 d5 6. d4 c6 7. Qc2 b6 8. Rd1 Nbd7 9. b3 Bb7 10. Nc3 Rc8 11. e4 dxe4 12. Nxe4 Nxe4 13. Qxe4 Qc7 14. Bf4 Bd6 15. Bxd6 Qxd6 16. c5 Qe7 (16... bxc5? 17. dxc5 Nxc5? {This makes it worse} 18. Qb4 Qe7 19. Rdc1 $18) 17. b4 Rfd8 18. Qc2 Nf6 19.Ne5 Nd5 20. a3 b5 21. Be4 g6 22. h4 a5 23. bxa5 Ra8 24. a4 b4 25. Bxd5 Rxd5 26. Rab1 f6 27. Ng4 Rad8 28. Rxb4 Bc8 29. Ne3 R5d7 30. Rb8 e5 31. d5 cxd5 32. c6 Rd6 33. c7 Rf8 34. Rxd5 f5 35. Qc5 Re6 36. Qxe7 Rxe7 37. Rxc8 Rxc8 38. Rd8+ Re8 39. Rxe8+ Rxe8 40. Nd5 1-0
</code></pre>
