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a chess program developed and written by [[Rudolf Huber]] in [[C]]. In its early times in the mid 90s, SOS running on various platforms and operating systems had an own futuristic [[GUI|graphical user interface]]. SOS supported the [[Chess Engine Communication Protocol]] <ref>[http://www.open-aurec.com/wbforum/viewtopic.php?f=18&t=30441 New Winboard engine, SOS by Rudolf Huber, Germany !] by [[Frank Quisinsky]], [[Computer Chess Forums|Winboard Forum]], 27 October 1999</ref> , was available as [[ChessBase#YoungTalents|Young Talent]] by [[ChessBase]] running under the [[Fritz#FritzGUI|Fritz6 GUI]] <ref>[http://www.chessbase.com/support/support.asp?pid=100 Support - ChessBase, May 28th, 2000]</ref> , and since Rudolf is co-designer of the protocol, it finally changed to [[UCI]] <ref>[https://www.stmintz.com/ccc/index.php?id=208295 The new UCI / WB GUI Arena is available with UCI Arena SOS ..] by [[Frank Quisinsky]], [[CCC]], January 18, 2002</ref> , and is a ''Partner Chess Engine'' of [[Arena]] <ref>[https://web.archive.org/web/20120103164848/http://www.playwitharena.com/?Partner_Chess_Engines Arena Chess GUI 3.0 - Partner Chess Engines]([https://en.wikipedia.org/wiki/Wayback_Machine Wayback Machine])</ref> <ref>[https://web.archive.org/web/20120106001947/http://www.playwitharena.com/?Partner_Chess_Engines:SOS%26nbsp%3B Arena Chess GUI 3.0 - SOS]([https://en.wikipedia.org/wiki/Wayback_Machine Wayback Machine])</ref> . SOS evolved from [[Principal Variation Search|PVS]] to [[MTD(f)]] and further as [[ParSOS]] to [[Parallel Search|parallel]] MTD(f).

SOS is a conventional chess program. It uses [[Depth-First|depth first]] [[Minimax|minimax tree search]] with [[Quiescence Search|quiescence search]], [[Alpha-Beta|alpha-beta]] enhancement, [[Principal Variation Search|minimal window search]] and [[Null Move Pruning|null-move pruning]]. To improve the search efficiency, the [[History Heuristic|history heuristic]] and a [[Transposition Table|transpositional table]] is used. The search is [[Extensions|extended]] to deeper plies on those move sequences which have a high probability of being part of the [[Principal Variation|principal variation]]. For SOS, those sequences are [[Recapture Extensions|recaptures]] and [[Check Extensions|check evasions]]. Leaf node [[Evaluation|evaluation]] considers only [[Material|material]], [[Piece-Square Tables|piece placement]] and [[Pawn Structure|pawn structure]] and only about 10% of the CPU time is spent on this (not including the quiescence search which is capture only, but extends on "losing" captures which are checks and on checking sequences). The evaluation parameters are dynamic and [[Incremental Updates|continuously updated]] during tree search. SOS's weakest part is probably [[Endgame|endgame]] knowledge. SOS actively plays a wide range of [[Opening|openings]], but most of those lines are not very deep. With autoplay games against itself, the [[Opening Book|opening book]] is tuned to favour favor those lines which harmonize with SOS's style of play.

SOS is an amateur program which was started in 1993 and has since then competed in a number of [[Tournamentsand Matches|tournaments]]. The newest version runs on multiprocessor systems with a parallelized version of [[MTD(f)|mtd(f)]] as its minimax search algorithm. SOS used to be a relatively fast searcher and relied on outsearching the opponent. This has changed now and more [[Knowledge|knowledge]] and special cases have been implemented which slow it down. Little effort is spent on the opening book. It plays a very broad range of openings. However it [[Book Learning|learns]] to avoid unsuccessful lines and tries not to repeat lost games. It uses publicly available [[Endgame Tablebases|endgame databases]].

* [https://web.archive.org/web/20120106001947/http://www.playwitharena.com/?Partner_Chess_Engines:SOS%26nbsp%3B Arena Chess GUI 3.0 - SOS]([https://en.wikipedia.org/wiki/Wayback_Machine Wayback Machine])* [https://web.archive.org/web/20120106031235/http://www.playwitharena.com/?Newsticker:Archive_9 Interview with SOS programmer Rudolf Huber in German language!] by [[Frank Quisinsky]], [[Arena|Arena Chess GUI 3.0]] - Archive 9, 132, May 10, 2005([https://en.wikipedia.org/wiki/Wayback_Machine Wayback Machine])* [http://scleinzell.schachvereine.de/p_spielprogramme/youngtal_b.shtml Young Talents, Teil 2] by [[Peter Schreiner]], Mai 2000, hosted by [http://scleinzell.schachvereine.de/home/news.shtml Schachclub Leinzell] (German)

* [http://www.newinchess.com/SOS/Default.aspx New In Chess - Secrets of Opening Surprises (SOS)] by [http://www.newinchess.com/Archives/ArticlesList.aspx?AuthorID=355 Jeroen Bosch]* [https://en.wikipedia.org/wiki/Buchholz_system Buchholz_system Buchholz system - SOS: Sum of Opponent Scores from Wikipedia]

* [https[:Category://en.wikipedia.org/wiki/Wes_Montgomery Wes Montgomery|Wes Montgomery]] - S.O.S. (take 3), [https://en.wikipedia.org/wiki/Full_House_(Wes_Montgomery_album) Full House], recorded at [https://en.wikipedia.org/wiki/The_Jabberwock_(club) Tsubo], [https://en.wikipedia.org/wiki/Berkeley,_California Berkeley, California], June 25, 1962 , [https://en.wikipedia.org/wiki/YouTube YouTube] Video> : feat. [https://en.wikipedia.org/wiki/Johnny_Griffin Johnny Griffin], [https://en.wikipedia.org/wiki/Wynton_Kelly Wynton Kelly], [https://en.wikipedia.org/wiki/Paul_Chambers Paul Chambers], [https://en.wikipedia.org/wiki/Jimmy_Cobb Jimmy Cobb]