Editing Reversi (section)

== Rules ==
Each of the disks' two sides corresponds to one player; they are referred to here as ''light'' and ''dark'' after the sides of ''Othello'' pieces, but any counters with distinctive faces are suitable. The game may for example be played with a chessboard and [[Scrabble]] pieces, with one player ''letters'' and the other ''backs''.

The historical version of reversi starts with an empty board, and the first two moves made by each player are in the four central squares of the board. The players place their disks alternately with their colors facing up and no captures are made. A player may choose to not play both pieces on the same diagonal, different from the standard ''Othello'' opening. It is also possible to play variants of Reversi and ''Othello'' where the second player's second move may or must flip one of the opposite-colored disks (as variants closest to the normal games).

For the specific game of ''Othello'', the game begins with four disks placed in a square in the middle of the grid, two facing light-side-up, two dark-side-up, so that the same-colored disks are on a diagonal. Convention has this such that the dark-side-up disks are to the north-east and south-west (from both players' perspectives), though this is only marginally consequential: where sequential openings' memorization is preferred, such players benefit from this. The dark player moves first.
{{Reversi
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| | | | | | | | 
| | | | | | | | 
| | | | | | | | 
| | | |O|X| | | 
| | | |X|O| | | 
| | | | | | | | 
| | | | | | | | 
| | | | | | | | 
| {{center|Starting position}}
}}
Dark must place a piece (dark-side-up) on the board and so that there exists at least one straight (horizontal, vertical, or diagonal) occupied line between the new piece and another dark piece, with one or more contiguous light pieces between them. For move one, dark has four options shown by translucently drawn pieces below:
{{Reversi
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| | | | | | | | 
| | | |x| | | | 
| | |x|O|X| | | 
| | | |X|O|x| | 
| | | | |x| | | 
| | | | | | | | 
| | | | | | | | 
| {{center|Where dark may play}}
}}
Play always alternates unless one player has no legal move, in which case they pass. After placing a dark disk, dark turns over (flips to dark, captures) the single disk (or chain of light disks) on the line between the new piece and an anchoring dark piece. Multiple chains of disks may be captured in a single move. No player can look back to the previous status of disks when playing moves. A valid move is one where at least one piece is reversed (flipped over).

If dark decided to put a piece in the topmost location (all choices are strategically equivalent at this time), one piece gets turned over, so that the board appears thus:
{{Reversi
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| | | | | | | | 
| | | |X| | | | 
| | | |X|X| | | 
| | | |X|O| | | 
| | | | | | | | 
| | | | | | | | 
| | | | | | | | 
| {{center|After dark play}}
}}
Now light plays. This player operates under the same rules, with the roles reversed: light lays down a light piece, causing a dark piece to flip. Possibilities at this time appear thus (indicated by transparent pieces):
{{Reversi
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| | | | | | | | 
| | | | | | | | 
| | |o|X|o| | | 
| | | |X|X| | | 
| | |o|X|O| | | 
| | | | | | | | 
| | | | | | | | 
| | | | | | | | 
| {{center|Where light may play}}
}}
Light takes the bottom left option and reverses one piece:
{{Reversi
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| | | | | | | | 
| | | | | | | | 
| | | |X| | | | 
| | | |X|X| | | 
| | |O|O|O| | | 
| | | | | | | | 
| | | | | | | | 
| | | | | | | | 
| {{center|After light play}}
}}
Players take alternate turns. If one player cannot make a valid move, play passes back to the other player. The game ends when the grid has filled up or if neither player can make a valid move.

Examples where the game ends before the grid is completely filled:
{{Reversi
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|O|O|O|O|O|O|O|O
|O|O|O|O|O|O|O|O
|O|O|O|O|O|O|O|O
|O|O|O|O|O|O|O| 
|O|O|O|O|O|O| | 
|O|O|O|O|O|O| |X
|O|O|O|O|O|O|O| 
|O|O|O|O|O|O|O|O
| {{center|Vlasáková 1 – 63 Schotte (European Grand Prix Prague 2011)}}
}}

{{Reversi
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||X|X|X|X|X|X|X
||O|O|O|O|O||X
|O|O|O|O|O|O|O|X
|O|O|O|O|O|O|O|X 
|O|O|O|O|O|O|O|X 
|O|O|O|O|O|O|O|X
|O|O|O|O|O|O|O|X 
||O|O|O|O|O||
| {{center|Vecchi 13 – 51 Nicolas (World Othello Championship 2017, Ghent)}}
}}

{{Reversi
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|||||O|||
|||||O|O||
|O|O|O|O|O|O|O|X
|||O|O|O|O||X 
|||O|O|O|||X 
||||||||
|||||||| 
||||||||
| {{center|Hassan 3 – 61 Verstuyft J. (European Grand Prix Ghent 2017)}}
}}

The player with the most pieces on the board at the end of the game wins. The game is scored by counting the number of discs in each player's color. If the game ended before the grid was completely filled, any empty squares are scored for the winner.<ref>{{Cite web |date=2019 |title=World Othello Championships Rules |url=https://www.worldothello.org/application/files/8816/9235/2069/NEW_WOC_Rules_2023.pdf |access-date=4 December 2024 |website=World Othello Federation}}</ref> However, if one player defaults by running out of time, that player's opponent wins regardless of the board configuration. There are varying methods to determine the official score when a player defaults.

In common practice over the Internet, opponents agree upon a time-control of, typically, from one to thirty minutes per game per player. Standard time control in the World Championship is thirty minutes, and this or something close to it is common in over-the-board (as opposed to internet) tournament play generally. In time-defaulted games, where disk differential is used for tie-breaks in tournaments or for rating purposes, one common over-the-board procedure for the winner of defaulted contests to complete both sides' moves with the greater of the result thereby or one disk difference in the winner's favor being the recorded score. Games in which both players have the same number of disks their color at the end (almost always with a full-board 32–32 score) are not very common, but also not rare, and these are designated as 'ties' and scored as half of a win for each player in tournaments. The term 'draw' for such may also be heard, but is somewhat frowned upon.

What are generally referred to as ''transcript sheets'' are generally in use in tournament over-the-board play, with both players obligated to record their game's moves by placing the number of each move in an 8×8 grid. This both enables players to look up past games of note and tournament directors and players to resolve disputes (according to whatever specific rules are in place) where claims that an illegal move, flip or other anomaly are voiced. An alternative recording method not requiring a grid is also in use, where positions on a board are labeled left to right by letters ''a'' through ''h'' and top to bottom by digits ''1'' through ''8''. Note that the numbers run in the opposite direction to [[Algebraic notation (chess)|the chess standard]], and that the perspective may be that of either player (with no fixed standard). This alternate notational scheme is used primarily in verbal discussions or where a linear representation is desirable in print, but may also be permissible as during-game transcription by either or both players.

Tournament over-the-board play has various ways of handling illegal moves and incorrect flips. For example, one procedure that has been used is to permit either player to make corrections going back some fixed number of moves.

=== Anti-Reversi ===
''Anti-Reversi'' or ''Reversed Reversi'' is a variant of the game where the player wins who has ''fewer'' own-colored disks at the end of the game; draw is also a possible result.<ref>About Anti-Reversi https://samsoft.org.uk/reversi/strategy.htm on 14 August 2024</ref> To put a disk on the board, the same rules apply as in normal reversi.

=== Brightwell Quotient ===
Invented by the British mathematician and three times runner-up at the World Championship and five times British Champion [[Graham Brightwell]], this is the tie-breaker that is now used in many tournaments including the W.O.C. If two players have the same number of points in the thirteen-round W.O.C. Swiss, the tie is resolved in favour of the player with the higher Brightwell Quotient.{{cn|date=June 2024}}

The Brightwell Quotient (BQ) is calculated as follows:<ref>{{Cite web |title=Britishothello.org.uk |url=http://britishothello.org/wp-content/uploads/2018/08/BOFRules.pdf |url-status=dead |archive-url=https://web.archive.org/web/20190218021133/http://britishothello.org/wp-content/uploads/2018/08/BOFRules.pdf |archive-date=18 February 2019}}</ref>
# A constant ''c'' is calculated. It is the integer nearest to (number of squares on the board) divided by (number of rounds in the tournament).
# If any of the player's opponents have withdrawn in the course of the tournament, or if a player has been paired against ''[[Bye (sports)|bye]]'', ignore such games for the moment. 
# Calculate the total number of disks scored by the player in all games not covered by step 2 and add ''c'' times the sum of points scored by all of the player's opponents, except those who have withdrawn.
# For each game against an opponent who has withdrawn, and each bye received, add half the number of squares on the board plus (''c'' times the player's own tournament score) to the result calculated in step 3. The number resulting is the player's BQ.