# Piece Designators

A*piece designator*is a set filter which represents the squares on which certain pieces lie in the current position. Piece designators are the foundation of CQL files: nearly every CQL file includes them. Examples of piece designators are:

Piece designator example | denoted squares |
---|---|

`b2` | square b2 |

`Q` | squares on which there is a white queen |

`a-h3` | squares on 3rd rank |

`[Qq]` | squares on which there is a white or black queen |

`Qa3` | a3, if there is a white queen on a3, otherwise no squares |

`[PqR]a-h2` | squares on the 2nd rank on which there is either a white pawn, a black queen, or a white rook |

`A[b-h2,d5,e-f7-8]` | squares in b2,c2,d2,e2,f2,g2,h2,d5,e7,f7,e8,f8 on which there is a white piece |

`_f1-8` | empty squares on the f file |

*piece type designator*followed by a

*square designator*. If the piece type designator is absent, all piece types (including empty square) are denoted. If the square designator is absent, all 64 squares are denoted.

## The piece type designator

A piece type designator is either a*simple piece type designator*or a

*compound piece type designator*. The simple piece type designators are these:

designator | meaning |
---|---|

`Q` | white queen |

`q` | black queen |

`B` | white bishop |

`b` | black bishop |

`R` | white rook |

`r` | black rook |

`N` | white knight |

`n` | black knight |

`K` | white king |

`k` | black king |

`P` | white pawn |

`p` | black pawn |

`A` | any white piece |

`a` | any black piece |

`_` | empty square |

A *compound piece type designator* is a string each of whose characters are simple piece type designators, enclosed in square brackets.

Examples of piece type designators are:

compound piece designator | meaning |
---|---|

`[Ak]` | Any white piece or the black king |

`[KQRBNPk]` | Any white piece or the black king |

`[K_]` | white king or empty square |

`[Aa_]` | any piece or empty square |

## The square designator

A*square designator*is a

*simple square designator*or a

*compound square designator*. A

*simple square designator*is a

*file designator*followed by a

*rank designator*(without white space between them). A

*file designator*is a

*file name*optionally followed by a hyphen and a file name. A

*file name*is one of the characters

`a`

through `h`

.
A *rank designator* is a number between `1`

and `8`

, or two such numbers separated by a hyphen.
Examples of simple square designators are:

simple square designator |
---|

`d4` |

`d4-5` |

`a-h1-8` |

A *compound square designator* is a list of simple square designators separated by commas without whitespace and enclosed in square brackets, such as:

compound square designator |
---|

`[a1,h1,a8,h8]` |

`[a1-8,a-h1,a-h8,h1-8]` |

Note that square brackets `[]`

are used to enclose piece types in a compound piece type designator and to enclose squares in a compound square designator.
A simple square designator denotes the squares on the chessboard that it names in the obvious way.

A compound square designator denotes the union of the squares denoted by the simple piece designators that it contains.
For example, `[a1,h1,a8,h8]`

denotes the corners of the board. `[a1-8,a-h1,a-h8,h1-8]`

denotes the edges of the board.

Note: compound square designators are rarely used in CQL 6. It is usually clearer to use the | symbol to denote union of the sets of squares.

Thus, instead of `[a1,h1,a8,h8]`

, you can use

{a1|h1|a8|h8}

This is a compound filter filter denoting the set containing the four indicated squares.

## Piece designators denote squares

In CQL, we think of piece designators as denoting sets of squares in the current position.

- A simple piecetype designator denotes the square on which the a piece (or empty square) of the kind it describes lie.
- A compound piecetype designator denotes the union of the squares denoted by its constituent simple piecetype designators.

For example, if the current position is the *start position* of a chess game, then the the piece designators in the left column below denote the sets of square in the right column:

piece designator | squares denoted in start position |
---|---|

`R` | the two squares `[a1,h1]` |

`[Rr]` | the four corner squares |

`Ra1` | the square a1 |

`Ra2` | the empty set of squares. |

`[Pp]d-e1-8` | the four squares [d2,e2,d7,e7] |

`_` | the squares [a-h3-6] |

`[nb]` | the squares [b-c8,f-g8] |