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A '''chess problem''', also called a '''chess composition''', is a [[Chess puzzle|puzzle]] set by the composer using [[chess piece]]s on a [[chess]] board, which presents the solver with a particular task. For instance, a position may be given with the instruction that White is to move first, and [[checkmate]] Black in two moves against any possible defence. A chess problem fundamentally differs from {{chessgloss|over-the-board}} play in that the latter involves a struggle between Black and White, whereas the former involves a competition between the composer and the solver. Most positions which occur in a chess problem are 'unrealistic' in the sense that they are very unlikely to occur in over-the-board play.<ref name=oz>{{Cite web|url=http://www.ozproblems.com/home|title=OzProblems - Australian chess problem composition|website=www.ozproblems.com}}</ref> There is a good deal of [[Glossary of chess problems|specialized jargon used in connection with chess problems]].
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*The solution should illustrate a theme or themes, rather than emerging from disjointed calculation. Many of the more common themes have been given names by problemists (see [[chess problem terminology]] for a list).
*The key move of the solution should not be obvious. Obvious moves such as checks, captures, and (in directmates) moves which restrict the movement of the black king make for bad keys. Keys which deprive the black king of some squares to which it could initially move (<em>flight squares</em>), but at the same time make available an equal or greater number of flight squares are acceptable. Key moves which prevent the enemy from playing a checking move are also undesirable, particularly in cases where there is no mate provided after the checking move. In general, the weaker (in terms of ordinary over-the-board play) the key move is the less obvious it will be, and hence the more highly prized it will be.
*There should be no [[promotion (chess)|promoted]] pawns in the initial position. For example, if White has three knights, one of them must clearly have been promoted; the same is true of two {{chessgloss|light-square bishops}}. There are more subtle cases: if f1 is empty, a white bishop stands on b5 and there are white pawns on e2 and g2, then the bishop must be a promoted pawn (there is no way the original bishop could have gotten past those unmoved pawns). A piece such as this, which does not leave a player with pieces additional to those at the start of a game, but which nonetheless must have been promoted, is called <em>obtrusive</em>. The presence of obtrusive units constitutes a smaller flaw than the presence of more obviously promoted units.
*The problem should be economical.<ref>Iqbal, A. (2008). Evaluation of Economy in a Zero-sum Perfect Information Game, The Computer Journal, Oxford University Press, Vol. 51, No. 4, pp. 408–418, {{doi|10.1093/comjnl/bxm060}}. Online ISSN 1460-2067, Print ISSN 0010-4620. http://comjnl.oxfordjournals.org/content/51/4/408.abstract {{Webarchive|url=https://web.archive.org/web/20160608122007/http://comjnl.oxfordjournals.org/content/51/4/408.abstract |date=2016-06-08 }}</ref> There are several facets to this desideratum. For one thing, every piece on the board should serve a purpose, either to enable the actual solution, or to exclude alternative solutions. Extra units should not be added to create "red herrings" (this is called <em>dressing the board</em>), except in rare cases where this is part of the theme. If the theme can be shown with fewer total units, it should be. For another, the problem should not employ more moves than is needed to exhibit the particular theme(s) at its heart; if the theme can be shown in fewer moves, it should be.
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