Chess960 glossary

The terms you meet in Chess960 and cannot easily look up: two rival numbering and notation schemes, an engine flag that silently ruins analysis, and the handful of constraints that produce exactly 960 positions.

By Carl Brodsky. Where a count is given below, it was checked against the generator this site runs.

Chess960 terms, defined

Back rank

The eight squares a player's pieces occupy at the start, rank 1 for White and rank 8 for Black. It is the only thing Chess960 changes: pawns, board, moves and checkmate are untouched, so everything unfamiliar in a game traces back to those eight squares.

Bishop-colour constraint

The requirement that the two bishops start on opposite-coloured squares. Drop it and arrays with two light-squared bishops become legal, leaving every dark square permanently unreachable by a bishop for the whole game, which is a different game rather than a shuffled one.

Castling rights in FEN

Standard FEN writes castling rights as KQkq, where K means "the kingside rook." With the rooks shuffled, "kingside" names nothing until you already know where the rooks are, so Chess960 formats identify each rook by its file instead: HAha says White may still castle with the h-rook and the a-rook, and lowercase says the same for Black. Uppercase is always White.

Chess960

Chess with the back rank shuffled into one of 960 legal arrangements before play, Black's array reflecting White's file for file. Same pieces, same moves, same checkmate; castling is the only rule that has to be restated, and the restatement is on the rules page.

Corner rook problem

Not a rule, a practical complaint: an array can leave a rook on a1 or h1 with its own pieces in front of it and no file to open, and unlike standard chess, castling may not rescue it. In eighteen of the 960 arrays the king starts on g1 with rooks on f1 and h1, so castling short wants to put the h-rook on a square the other rook is already sitting on, and one of them has to move first.

Double fianchetto start

Any of the sixty arrays with a bishop on b1 and a bishop on g1. Both bishops already point along the long diagonals before anything has moved, and one pawn step apiece puts them there properly, which makes these among the fastest positions in Chess960 to develop and among the hardest to attack early.

Fischer Random

Bobby Fischer's name for the variant, announced in 1996, and the same game as Chess960. Which name you meet depends on where you are: FIDE uses Fischer Random for its world championship, while sites, books and engines mostly use Chess960.

Freestyle Chess

A recent circuit of elite invitational tournaments played from shuffled starting positions, and the name its organisers use for the game itself. It is why Chess960 results now appear in general chess coverage rather than only in variant corners. See also the Chess960 ratings page for how those events sit beside the rating lists.

King-between-rooks constraint

The requirement that the king starts on a square with a rook on either side. It exists to make castling definable: with one rook to the left and one to the right, "castle short" and "castle long" always name a real move, in every one of the 960 arrays.

King-takes-rook castling

The convention of entering a castling move by playing the king onto its own rook rather than onto its destination square. It exists because the destination is ambiguous: in ninety of the 960 arrays the king starts on f1 with a rook on g1, and "king to g1" could equally be a plain king step or castling short. Nearly every interface and every engine protocol uses it.

Mirrored setup

Two different things wear this name. Black's setup always mirrors White's across the board, file for file, so neither side gets a structural edge; and separately, any array reflected left to right is itself a legal array with a different number, the reflection of position 518 being 534. No array is its own reflection, because the king would have to stand on a square that reflects to itself, and a rank of eight squares has none.

O-O and O-O-O

Castling notation, unchanged from standard chess. O-O finishes with the king on g1 and the rook on f1, O-O-O with the king on c1 and the rook on d1, whatever squares the two pieces started on. That is why O-O can move only the rook, or only the king, or carry the king leftwards.

Opening book

A stored list of opening moves an engine or a player plays from memory instead of calculating. In Chess960 there is nothing worth storing for 959 of the 960 positions, so engines calculate from move one and so do you.

Position 518

The standard chess array, RNBQKBNR, under the usual numbering. It is the one starting position out of 960 where opening theory applies, and its presence in the set is the reason Chess960 is described as a superset of chess rather than a variant of it. Step to it in the position generator and the board is the one you know.

Shredder-FEN

A FEN dialect that always writes castling rights as rook files, so the ordinary chess start is written HAha rather than KQkq. Prefer it when a string has to survive being read by something you did not write, because it never requires interpretation.

SP number

Starting position number, 0 to 959, in the Scharnagl numbering that lichess, chess.com and FIDE material all share. Quote an SP number and the other player has your exact board. The derivation places the light-squared bishop, then the dark-squared bishop, then the queen, then reads the last five squares off a fixed table of ten king-rook-knight patterns, and that table is what guarantees the king lands between the rooks.

Starting array

The eight letters describing one back rank, read from the a-file to the h-file, as in BBQNNRKR. It is the quickest unambiguous way to write a position down when the number is not to hand.

Transposition

Reaching an identical position by a different order of moves. In standard chess it is a preparation tool, since move orders funnel into named openings; in Chess960 there is nothing named to funnel into, and two different arrays cannot produce the same position until enough pieces have moved or been captured to erase the difference, so the idea nearly disappears.

UCI_Chess960

A boolean option in the UCI protocol that tells an engine the game is Chess960. Left off, the engine reads king-takes-rook moves as ordinary king moves or rejects them, and may offer castling that is illegal in the array on the board. If engine analysis of a shuffled position looks nonsensical, check this flag before anything else.

X-FEN

The other FEN dialect. It keeps KQkq and defines K as the outermost rook on that side of the king, falling back to a file letter only when that reading would be wrong, which happens after a promotion leaves a second rook on the back rank. So X-FEN and Shredder-FEN agree on almost everything and disagree on how the ordinary start is spelled.

Where to use these

The vocabulary is worth about ten minutes; the skill it describes takes longer. Play a shuffled game against the engine on the home board, drill the tactics on unfamiliar squares in the Chess960 puzzle trainer, or work through a thousand of them in the Chess960 puzzle books.