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## Overview
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[](https://travis-ci.org/official-stockfish/Stockfish)
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[](https://ci.appveyor.com/project/mcostalba/stockfish/branch/master)
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[Stockfish](https://stockfishchess.org) is a free, powerful UCI chess engine
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derived from Glaurung 2.1. It is not a complete chess program and requires a
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UCI-compatible GUI (e.g. XBoard with PolyGlot, Scid, Cute Chess, eboard, Arena,
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Sigma Chess, Shredder, Chess Partner or Fritz) in order to be used comfortably.
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Read the documentation for your GUI of choice for information about how to use
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Stockfish with it.
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## Files
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This distribution of Stockfish consists of the following files:
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* Readme.md, the file you are currently reading.
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* Copying.txt, a text file containing the GNU General Public License version 3.
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* src, a subdirectory containing the full source code, including a Makefile
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that can be used to compile Stockfish on Unix-like systems.
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## UCI parameters
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Currently, Stockfish has the following UCI options:
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* #### Debug Log File
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Write all communication to and from the engine into a text file.
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* #### Contempt
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A positive value for contempt favors middle game positions and avoids draws.
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* #### Analysis Contempt
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By default, contempt is set to prefer the side to move. Set this option to "White"
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or "Black" to analyse with contempt for that side, or "Off" to disable contempt.
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* #### Threads
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The number of CPU threads used for searching a position. For best performance, set
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this equal to the number of CPU cores available.
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* #### Hash
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The size of the hash table in MB. It is recommended to set Hash after setting Threads.
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* #### Clear Hash
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Clear the hash table.
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* #### Ponder
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Let Stockfish ponder its next move while the opponent is thinking.
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* #### MultiPV
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Output the N best lines (principal variations, PVs) when searching.
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Leave at 1 for best performance.
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* #### Skill Level
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Lower the Skill Level in order to make Stockfish play weaker (see also UCI_LimitStrength).
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Internally, MultiPV is enabled, and with a certain probability depending on the Skill Level a
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weaker move will be played.
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* #### UCI_LimitStrength
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Enable weaker play aiming for an Elo rating as set by UCI_Elo. This option overrides Skill Level.
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* #### UCI_Elo
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If enabled by UCI_LimitStrength, aim for an engine strength of the given Elo.
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This Elo rating has been calibrated at a time control of 60s+0.6s and anchored to CCRL 40/4.
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* #### UCI_ShowWDL
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If enabled, show approximate WDL statistics as part of the engine output.
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These WDL numbers model expected game outcomes for a given evaluation and
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game ply for engine self-play at fishtest LTC conditions (60+0.6s per game).
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* #### Move Overhead
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Assume a time delay of x ms due to network and GUI overheads. This is useful to
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avoid losses on time in those cases.
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* #### Slow Mover
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Lower values will make Stockfish take less time in games, higher values will
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make it think longer.
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* #### nodestime
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Tells the engine to use nodes searched instead of wall time to account for
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elapsed time. Useful for engine testing.
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* #### UCI_Chess960
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An option handled by your GUI. If true, Stockfish will play Chess960.
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* #### UCI_AnalyseMode
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An option handled by your GUI.
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* #### SyzygyPath
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Path to the folders/directories storing the Syzygy tablebase files. Multiple
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directories are to be separated by ";" on Windows and by ":" on Unix-based
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operating systems. Do not use spaces around the ";" or ":".
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Example: `C:\tablebases\wdl345;C:\tablebases\wdl6;D:\tablebases\dtz345;D:\tablebases\dtz6`
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It is recommended to store .rtbw files on an SSD. There is no loss in storing
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the .rtbz files on a regular HD. It is recommended to verify all md5 checksums
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of the downloaded tablebase files (`md5sum -c checksum.md5`) as corruption will
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lead to engine crashes.
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* #### SyzygyProbeDepth
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Minimum remaining search depth for which a position is probed. Set this option
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to a higher value to probe less agressively if you experience too much slowdown
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(in terms of nps) due to TB probing.
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* #### Syzygy50MoveRule
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Disable to let fifty-move rule draws detected by Syzygy tablebase probes count
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as wins or losses. This is useful for ICCF correspondence games.
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* #### SyzygyProbeLimit
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Limit Syzygy tablebase probing to positions with at most this many pieces left
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(including kings and pawns).
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## What to expect from Syzygybases?
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If the engine is searching a position that is not in the tablebases (e.g.
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a position with 8 pieces), it will access the tablebases during the search.
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If the engine reports a very large score (typically 153.xx), this means
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that it has found a winning line into a tablebase position.
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If the engine is given a position to search that is in the tablebases, it
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will use the tablebases at the beginning of the search to preselect all
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good moves, i.e. all moves that preserve the win or preserve the draw while
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taking into account the 50-move rule.
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It will then perform a search only on those moves. **The engine will not move
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immediately**, unless there is only a single good move. **The engine likely
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will not report a mate score even if the position is known to be won.**
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It is therefore clear that this behaviour is not identical to what one might
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be used to with Nalimov tablebases. There are technical reasons for this
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difference, the main technical reason being that Nalimov tablebases use the
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DTM metric (distance-to-mate), while Syzygybases use a variation of the
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DTZ metric (distance-to-zero, zero meaning any move that resets the 50-move
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counter). This special metric is one of the reasons that Syzygybases are
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more compact than Nalimov tablebases, while still storing all information
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needed for optimal play and in addition being able to take into account
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the 50-move rule.
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## Large Pages
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Stockfish supports large pages on Linux and Windows. Large pages make
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the hash access more efficient, improving the engine speed, especially
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on large hash sizes. Typical increases are 5..10% in terms of nps, but
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speed increases up to 30% have been measured. The support is
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automatic. Stockfish attempts to use large pages when available and
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will fall back to regular memory allocation when this is not the case.
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### Support on Linux
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Large page support on Linux is obtained by the Linux kernel
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transparent huge pages functionality. Typically, transparent huge pages
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are already enabled and no configuration is needed.
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### Support on Windows
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The use of large pages requires "Lock Pages in Memory" privilege. See
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[Enable the Lock Pages in Memory Option (Windows)](https://docs.microsoft.com/en-us/sql/database-engine/configure-windows/enable-the-lock-pages-in-memory-option-windows)
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on how to enable this privilege. Logout/login may be needed
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afterwards. Due to memory fragmentation, it may not always be
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possible to allocate large pages even when enabled. A reboot
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might alleviate this problem. To determine whether large pages
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are in use, see the engine log.
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## Compiling Stockfish yourself from the sources
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Stockfish has support for 32 or 64-bit CPUs, certain hardware
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instructions, big-endian machines such as Power PC, and other platforms.
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On Unix-like systems, it should be easy to compile Stockfish
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directly from the source code with the included Makefile in the folder
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`src`. In general it is recommended to run `make help` to see a list of make
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targets with corresponding descriptions.
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```
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cd src
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make help
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make build ARCH=x86-64-modern
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```
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When not using the Makefile to compile (for instance with Microsoft MSVC) you
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need to manually set/unset some switches in the compiler command line; see
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file *types.h* for a quick reference.
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When reporting an issue or a bug, please tell us which version and
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compiler you used to create your executable. These informations can
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be found by typing the following commands in a console:
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```
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./stockfish
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compiler
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```
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## Understanding the code base and participating in the project
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Stockfish's improvement over the last couple of years has been a great
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community effort. There are a few ways to help contribute to its growth.
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### Donating hardware
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Improving Stockfish requires a massive amount of testing. You can donate
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your hardware resources by installing the [Fishtest Worker](https://github.com/glinscott/fishtest/wiki/Running-the-worker:-overview)
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and view the current tests on [Fishtest](https://tests.stockfishchess.org/tests).
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### Improving the code
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If you want to help improve the code, there are several valuable resources:
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* [In this wiki,](https://www.chessprogramming.org) many techniques used in
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Stockfish are explained with a lot of background information.
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* [The section on Stockfish](https://www.chessprogramming.org/Stockfish)
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describes many features and techniques used by Stockfish. However, it is
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generic rather than being focused on Stockfish's precise implementation.
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Nevertheless, a helpful resource.
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* The latest source can always be found on [GitHub](https://github.com/official-stockfish/Stockfish).
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Discussions about Stockfish take place in the [FishCooking](https://groups.google.com/forum/#!forum/fishcooking)
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group and engine testing is done on [Fishtest](https://tests.stockfishchess.org/tests).
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If you want to help improve Stockfish, please read this [guideline](https://github.com/glinscott/fishtest/wiki/Creating-my-first-test)
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first, where the basics of Stockfish development are explained.
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## Terms of use
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Stockfish is free, and distributed under the **GNU General Public License version 3**
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(GPL v3). Essentially, this means that you are free to do almost exactly
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what you want with the program, including distributing it among your
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friends, making it available for download from your web site, selling
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it (either by itself or as part of some bigger software package), or
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using it as the starting point for a software project of your own.
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The only real limitation is that whenever you distribute Stockfish in
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some way, you must always include the full source code, or a pointer
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to where the source code can be found. If you make any changes to the
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source code, these changes must also be made available under the GPL.
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For full details, read the copy of the GPL v3 found in the file named
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*Copying.txt*.
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