Promotions are tactical moves changing material balance where a pawn reaches its promotion square on the eighth (or first) rank to transform to a piece of the players choice, either a queen, knight, rook or bishop of the same color. The pawn may reach the eighth rank due to a pawn push or capture. A pawn promoting to a queen while capturing the opponent queen defines the maximum amount of material a move may gain.
Since theoretical each pawn may promote, the number of pawns, eight, per side defines the maximum number of pieces, that is one side may have up to nine queens, or ten knights, as well as bishops or rooks. The maximum sum of these pieces, seven of the initial position, is fifteen.
Queening versus Minor Promotions
Most often in practical play promotions occur to the strongest piece, the queen, also referred as queening. A promotion to other than the queen is called minor- or underpromotion, where once in a while the promotion to a knight has some practical relevance due to its distinct attacking with the possibility to check  or to fork, see for instance what happened at ACM 1991 in Albuquerque to Lachex . Minor promotions to rook and bishop, whose attacks are sub-set of the queen are very rarely about to avoid stalemate - or may occur at the root in cases the promoted piece may be captured immediately anyway. In move encoding the promoted piece requires two extra bits.
In move generation it is quite common while producing the queen promotion, also to generate all the additional minor promotions, that is to write four moves to a move list with all two-bit combinations indicating the promoted piece, instead of one. However, some programs only generate queening and use unmake move of a queen promotion as trigger to conditionally generate further minor promotions, assuming the queen promotion didn't fail high or its refutation was not an immediate capture. Bishop and rook promotions may only generated if the queen promotion returns an explicit stalemate score. Other programs even omit minor promotions or at least underpromotions to rook and bishop inside their search except the root at all, with the motivation the reduced code and branching factor likely pays off in practical playing strength. In quiescence search most programs only consider queening.
Entering moves by a human operator within a graphical user interface requires modal interaction, for instance dragging the piece with a mouse from the departure square to a destination square. In case of promotions a modal dialog or pop-up menu is necessary to choose the promoted piece to finally terminate the modal move entering interaction.
- ACM 1991 | Mephisto - Lachex
- Algebraic Chess Notation - Promotions
- Entering Moves
- Famous Bugs
- HAKMEM 70
- Promotion Square
- Quiescence Search
- Tactical Moves
- Harold van der Heijden (1996). Pawn Promotion. New in Chess, Alkmaar ISBN 90-5691-005-1
- Glen Robert Downey (1998). The Truth About Pawn Promotion: The Development of the Chess Motif in Victorian Fiction. Ph.D. thesis, University of Victoria pdf » Alice 
- Underpromoting other than to a knight by David Fotland, CCC, January 29, 1998
- Pawn promotion defense by Scott Gasch, CCC, October 07, 1999
- Promotion frequency by Russell Reagan, CCC, July 23, 2002
- Promotion to Knight Wins by Rob Basham, CCC, May 22, 2004
- The Perils of missing sub-promotion by Nicolai Czempin, CCC, September 28, 2007
- Re: Botvinnik Markov revisited by Don Dailey, CCC, June 27, 2010 » Komodo
- Strange knight underpromotion by Critter by Richard Vida, CCC, March 04, 2010
- Wanted - Bishop promotion FEN by Steven Atkinson, CCC, March 07, 2012
- Some statistics about promotions and underpromotions by Jesús Muñoz, CCC, July 31, 2013
- Tough promotion bug(s) by Henk van den Belt, CCC, August 29, 2013
- Stockfish underpromotes much more often than Komodo by Kai Laskos, CCC, October 05, 2016 » Komodo, Match Statistics, Stockfish
- correct way to score promotions using MVV-LVA by Mahmoud Uthman, CCC, June 06, 2019 » MVV-LVA
- underpromoting is worth how many elo? by Warren D. Smith, FishCooking, March 08, 2020
- Under promotion of a pinned pawn to bishop by Oliver Brausch, CCC, September 03, 2020
- Promotion (chess) from Wikipedia
- Babson task from Wikipedia
- Saavedra position from Wikipedia
- Joe Henderson Quartet - Ya La Quiero, Burghausen 1987, YouTube Video