Promotion is actually very simple. When a pawn reaches the 8th rank, it is changed into a Queen, a Rook, a Bishop, or a Knight, according to the player’s choice. The new piece replaces the pawn on the same square.
Note: Promotion is not limited to pieces that have already been captured.
Promoting a pawn
This paragraph shows the different pawn promotions.
A promotion can be combined with a capture: the pawn moves diagonally as it would do in a normal capture.
Promoting to Queen or under-promoting ?
Practically speaking, the Queen being the strongest of all Chess pieces, promoting to a Queen is almost always the correct move.
Indeed, the Queen has the highest value amongst all Chess pieces. Its value is 9 (points), when the Rook is 5, and the Bishop and the Knight both 3. These values are approximate of course.
So, in most cases, the pawn must be promoted to Queen. This is the case of 97% of the promotions. Then the remaining promotions (by decreasing order of popularity) are the promotion to Knight, Rook and then Bishop. Promoting to another piece than the Queen is sometimes called underpromoting.
Promoting to a Rook is even rarer than promoting to a Knight. It is the right move only when promoting to a Queen would result in a Stalemate.
The endgame King+Rook VS King is winning for the side having the Rook, as there is a generic method to deliver checkmate with the Rook.
The last promotion is the promotion to a Bishop. Even if we could probably build a position where the promotion to Bishop would be the only correct move, this promotion is of little interest and you can probably assume you will never have to play this move.
This is all you need to know about promoting a pawn. The next chapters of our method to learn Chess are dedicated to the different ways to make a draw and to the Chess notation.