Underpromotion
When your pawn reaches the 7th rank, it feels almost instinctive to promote it to a queen; the queen is the most powerful piece, so it only makes sense to promote to it every time, right? Most of the time, yes, but not all of the time. Let’s take a look at a few positions where underpromotion is the better choice.
Since we’re so accustomed to promoting to a queen, your first instinct may have been to do just that. However, you may have noticed that if you promote to a queen, the black king has no squares to go to. Thus, a completely winning position for white would have turned into a stalemate.
We now know that promoting to a queen in this position isn’t a good idea. So would promoting to a rook be a better choice? Although it may look like a good choice, that too would cause a stalemate, as once again, the black king has no squares to go to.
If we had to promote in this position, the best pieces to promote to would be either the bishop or knight.
If white promotes to either of those pieces, the black king has no choice but to play Kb7 next move, and white will be able to maneuver towards a checkmate within the next couple of moves.
Let’s take a look at another example.
This position is a simple yet excellent demonstration of underpromotion. If white promotes the c7 pawn to a queen, the position is a stalemate. However, if white promotes the c7 pawn to a rook, the position is now a mate in one, as black must play Ka6 to which white plays Ra8#.
Let’s take a look at one more example.
If this position was white to play, and white had to promote, what would be acceptable pieces to promote to?
If you said the bishop or knight, that would be correct. Notice that if white promotes to a queen or rook, the black king has no squares to go to, and since the knight is pinned to the king by the queen/rook, black has no legal moves. Thus, promoting to a queen or rook would not work.
However, if white promotes to a bishop or knight, the black king does not have any squares to go to, but the black knight is free to move, and so it is not a stalemate.
Author: Saharsh Satheesh
