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'''Depth''' is the height or ''nominal'' depth in [[Ply|plies]] between the [[Root|root]] and so called [[Horizon Node|horizon nodes]] (depth 0), where a heuristic value is assigned to. Thus, depth is the number of half moves the search ''nominally'' looks ahead.
Despite [[Quiescence Search|quiescence search]], where usually winning captures and even some checks are tried at or behind the search horizon, until positions become sufficiently quite, [[Selectivity|selectivity]] of modern chess programs, caused by [[Extensions|extensions]], [[Pruning|pruning]] and [[Reductions|reductions]], notably [[Check Extensions|check extensions]], [[Null Move Pruning|nNMPNMP]] and [[Late Move Reductions|LMR]], leads to bushy, non-uniform [[Search Tree|trees]] where some branches are searched deeper than nominal, but others shallower. A [[Depth Reduction R|depth reduction R]] of multiple plies is often performed in forward pruning techniques like [[Null Move Pruning|null move pruning]] and [[Multi-Cut|multi-cut]].
=Draft versus Ply-Index=
<span id="FractionalPlies"></span>
=Fractional Plies=
Some programs extend or reduce in fractions of one ply. Fractional plies with integers, where one corresponds to 1/N, require a depth resolution of N. Inside an [[Iterative Deepening|iterative deepening]] framework, the search depth is incremented, usually by one ply , N - or by a fraction of one ply, for instance 1/2 ply.
[[Amir Ban]] on [[Junior]] in [[Computer Chess Forums|rgcc]], March 1998 <ref>[http://groups.google.com/group/rec.games.chess.computer/browse_frm/thread/2163f0fa2f0715f1 Funny Junior Engine in CBLight / Junior Engine ply depth] by Wolfgang Krietsch, [[Computer Chess Forums|rgcc]], February 27, 1998, post 7 and 16 by [[Amir Ban]]</ref> :
* [[Matej Guid]], [[Ivan Bratko]] ('''2007'''). ''Factors affecting diminishing returns for searching deeper''. [[ICGA Journal#30_2|ICGA Journal, Vol. 30, No. 2]], [http://www.ailab.si/matej/doc/Factors_Affecting_Diminishing_Returns.pdf pdf]
==2010 ...==
* [[Diogo R. Ferreira]] ('''2013'''). ''The Impact of the Search Depth on Chess Playing Strength''. [[ICGA Journal#36_2|ICGA Journal, Vol. 36, No. 2]]» [[#DiminishingReturns|Diminishing Returns]], [[Match Statistics]], [[Playing Strength]], [[Houdini]] <ref>[https://www.hiarcs.net/forums/viewtopic.php?t=10004 Ply versus ELO] by Greg, [[Computer Chess Forums|HIARCS Forum]], May 30, 2020 » [[Diogo R. Ferreira#Impact|Diogo R. Ferreira - Impact of the Search Depth ...]]</ref>
* [[Tamal T. Biswas]], [[Kenneth W. Regan]] ('''2015'''). ''Quantifying Depth and Complexity of Thinking and Knowledge''. [http://www.icaart.org/EuropeanProjectSpace.aspx?y=2015 ICAART 2015], [http://www.cse.buffalo.edu/~regan/papers/pdf/BiReICAART15CR.pdf pdf]
* [[Tamal T. Biswas]], [[Kenneth W. Regan]] ('''2015'''). ''Measuring Level-K Reasoning, Satisficing, and Human Error in Game-Play Data''. [[IEEE]] [http://www.icmla-conference.org/icmla15/ ICMLA 2015], [http://www.cse.buffalo.edu/~regan/papers/pdf/BiRe15_ICMLA2015.pdf pdf preprint]
* [[Matej Guid]], [[Ivan Bratko]] ('''2017'''). ''Influence of Search Depth on Position Evaluation''. [[Advances in Computer Games 15]]
=Forum PostsPostings= ==1983 ...==* [http://quux.org:70/Archives/usenet-a-news/NET.chess/82.01.07_alice.349_net.chess.txt chess strength] by [[Ken Thompson]], [http://quux.org:70/Archives/usenet-a-news/NET.chess net.chess], January 7, 1982
==1996 ...==
* [https://groups.google.com/d/msg/rec.games.chess.computer/1uVIWZFB41k/VUcAUkzyFd0J Fractional depth increments] by S. Read, [[Computer Chess Forums|rgcc]], January 18, 1996
* [http://www.talkchess.com/forum/viewtopic.php?t=63326 Ridiculous QSearch Depth] by [[Jonathan Rosenthal]], [[CCC]], March 03, 2017 » [[Quiescence Search]]
* [http://www.talkchess.com/forum3/viewtopic.php?f=7&t=70217 Depth reduced but ELO increased] by [[Tom King]], [[CCC]], March 16, 2019 » [[Countermove Heuristic]], [[Playing Strength]]
==2020 ...==
* [https://www.hiarcs.net/forums/viewtopic.php?t=10004 Ply versus ELO] by Greg, [[Computer Chess Forums|HIARCS Forum]], May 30, 2020 » [[Diogo R. Ferreira#Impact|Diogo R. Ferreira - Impact of the Search Depth ...]]
* [http://www.talkchess.com/forum3/viewtopic.php?f=7&t=77202 On reaching maximum ply] by [[Martin Bryant]], [[CCC]], April 29, 2021 » [[#MaxPly|Maximum Search Depth]], [[Ply]], [[Search]]
* [http://www.talkchess.com/forum3/viewtopic.php?f=2&t=77501 Ares playing strength at various fixed depth levels] by [[Charles Roberson]], [[CCC]], June 16, 2021 » [[Ares US|Ares]], [[Playing Strength]]
* [https://www.talkchess.com/forum3/viewtopic.php?f=7&t=79294 How calculate the seldepth] by ernesto, [[CCC]], February 06, 2022
=External Links=
