user17330: I have an impression that at the high level, the Pirc defense is considered to be much more off-beat than the King's Indian defense; at least the latter has been a main weapon for many top players, starting from Bronstein and Geller and through Fischer and Kasparov to Nakamura, while the Pirc is mostly seen when Black is in a desperate must-win situation.
However, I struggle to discern the reasons for that. The main difference seems to be that in the Pirc, White plays Nc3 with a pawn on c2 rather than c4, but isn't it a concession on their part? Doesn't having a pawn of c4 just give White more control over the center?

user924: The two openings don't really compete with each other, as one is a defense against 1.d4/c4/Nf3 and the other against 1.e4. To understand their popularity, you have to compare them against alternative choices that they do compete with.
So say black is looking for a defense to use as a main part of their repertoire when attempting to win as black, that yields sharp and unbalanced positions without giving white too much of a good position, even if white expects the opening.
Against 1.e4, there is the Sicilian, in various flavours. It probably scores the best for black of all mainstream chess openings. There is just little reason to choose the Pirc over the Sicilian if you are looking for a sharp and unbalanced winning attempt.
Against 1.d4 there are many choices but none are obviously better than the King's Indian. The Dutch and Benoni are much more risky, and the Grunfeld and Nimzo/QID are more solid than the KID in return for also giving fewer winning chances.
So there is a place in the repertoires of some strong GMs for the KID, but there is less reason to choose the Pirc. The Pirc's niche is as a surprise weapon.

user25320: The c4 square is a natural development square for White's light-squared bishop, which targets the f7-square. Having a pawn on c4 takes away that opportunity.
In semi-open defences against 1. e4 the f7-square is shielded by a pawn on e6. But this creates dark-square weaknesses in Black's position when they have a fianchettoed kingside bishop on g7. So Black has to be careful of liquidating pawns in the centre in the Pirc, so as not to expose the f7-square to the White bishop.
Liquidation of pawns in the centre is less of a problem in the King's Indian Defence, because White is then left with a pawn on c4 which hems in his light-squared bishop (denying it the c4-square). So White doesn't gain a natural advantage by liquidating the centre in the King's Indian, but it's a viable proposition in the Pirc.
These two key strategic reasons are why the Pirc is "less sound", or more difficult to play, than the King's Indian Defence.

17330: <p>I have an impression that at the high level, the Pirc defense is considered to be much more off-beat than the King's Indian defense; at least the latter has been a main weapon for many top players, starting from Bronstein and Geller and through Fischer and Kasparov to Nakamura, while the Pirc is mostly seen when Black is in a desperate must-win situation.</p>
<p>However, I struggle to discern the reasons for that. The main difference seems to be that in the Pirc, White plays Nc3 with a pawn on c2 rather than c4, but isn't it a concession on their part? Doesn't having a pawn of c4 just give White more control over the center?</p>
 924: <p>The two openings don't really compete with each other, as one is a defense against 1.d4/c4/Nf3 and the other against 1.e4. To understand their popularity, you have to compare them against alternative choices that they do compete with.</p>
<p>So say black is looking for a defense to use as a main part of their repertoire when attempting to win as black, that yields sharp and unbalanced positions without giving white too much of a good position, even if white expects the opening.</p>
<p>Against 1.e4, there is <strong>the Sicilian</strong>, in various flavours. It probably scores the best for black of all mainstream chess openings. There is just little reason to choose the Pirc over the Sicilian if you are looking for a sharp and unbalanced winning attempt.</p>
<p>Against 1.d4 there are many choices but none are obviously better than the King's Indian. The Dutch and Benoni are much more risky, and the Grunfeld and Nimzo/QID are more solid than the KID in return for also giving fewer winning chances.</p>
<p>So there is a place in the repertoires of some strong GMs for the KID, but there is less reason to choose the Pirc. The Pirc's niche is as a surprise weapon.</p>
 25320: <p>The c4 square is a natural development square for White's light-squared bishop, which targets the f7-square. Having a pawn on c4 takes away that opportunity.</p>
<p>In semi-open defences against 1. e4 the f7-square is shielded by a pawn on e6. But this creates dark-square weaknesses in Black's position when they have a fianchettoed kingside bishop on g7. So Black has to be careful of liquidating pawns in the centre in the Pirc, so as not to expose the f7-square to the White bishop.</p>
<p>Liquidation of pawns in the centre is less of a problem in the King's Indian Defence, because White is then left with a pawn on c4 which hems in his light-squared bishop (denying it the c4-square). So White doesn't gain a natural advantage by liquidating the centre in the King's Indian, but it's a viable proposition in the Pirc.</p>
<p>These two key strategic reasons are why the Pirc is &quot;less sound&quot;, or more difficult to play, than the King's Indian Defence.</p>
