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Chess related

This page lists the features that are directly chess related:

Chess rules

The chess rules are implemented completely, including all castling rules as well as “en passant”. Underpromotion is possible, both for the computer and the user. If the user does not enter a piece type when promoting, a queen will be selected automatically as this is the most common use case.

Draw by threefold repetition works, also if the repetition does not occur in successive moves. Draw by fifty moves is also incorporated. Besides, the evaluation will be gradually reduced towards draw after 20 full moves without capture or pawn move. This helps the software to recognise this tendency, especially in defence.


Starting colour: fixed (Black/White), random, human vs. human. Changing the colour is possible at any time using the “go” key, as well as switching to “human vs. human” during a game, which is useful for entering move sequences.

Playing strength

The playing strength is around 2100 ELO. This corresponds to “expert”, which is between the class A amateur players and the master levels. The CT800 is a formidable opponent for any hobby chess player.

The draw detection also prevents early draws. Below move 30, the CT800 stays clear of draw by repetition if there are suitable alternatives, i.e. if the CT800 thinks it is not down by more than 30 centipawns. That keeps human players from scoring an easy draw.

Eval noise

Blurred evaluation, in centipawns: off, light (±10), medium (±30), strong (±50). This is the amount of noise added to the evaluation in the middlegame (not in the endgame). More noise means more game variety, but at the cost of playing strength. On the other hand, this allows reducing the strength a bit.

Opening book

23,600 different plies in 13,400 unique positions. Transpositions are recognised.

If the player has White, but decides to logically play as Black (i.e. by intentionally losing a tempo), the computer will catch that. Some players try that for throwing the computer out of its opening book while still being able to use their own opening knowledge – but that fails if the computer also knows this trick.

The opening book can be switched off, except for White’s first ply (because of the time handling).

Closed positions are avoided in the opening book. After its end, positions with closed or blocked centre (c-f) are recognised and preferably avoided. This is a simple kind of anti-human-mode.

The selected moves are not always the “best” ones, they just have to be playable. The opening book favours variety over strength because this means more fun to the user. That is also why the book is not that deep, usually about four to five full moves. The design goal was rather breadth than depth.

However, not all the moves will be played actively – e.g. Grob’s Attack is only passive knowledge. Holds also for the Blackmar-Diemer-Gambit, which may even be declined and steered towards Caro-Kann or French Defence.


The CT800 knows the most important basic endgames:

  • Mating the lone king with king plus queen, rook, bishop pair or bishop and knight.
  • King and pawn vs. king (endgame bitbase). It does not run the risk of possibly slowing down the search, as it can happen with the “big” table bases on PCs. This endgame bitbase is just some nearly direct flash-ROM access. In fact, I have seen some test games where this bitbase made the decisive difference.
  • Draw with rim pawn and wrong bishop if the defending king controls the promotion square. Also when the rim pawn is a multiple pawn (doubled, tripled, ...).
  • Draw detection for king and two knights vs. king.
  • If one side has only a minor piece while the other still has pawns, then the latter will always be evaluated as better, even if the side with the minor piece is nominally up in material (e.g. king plus bishop vs. king plus two pawns).