Open file

The eight files are the board's vertical lines, labeled a through h. A file is open when it contains no White or Black pawns. Rooks and queens can move along it as long as no piece blocks their path.

An open file is valuable because it offers a route for pressure and possible entry into enemy territory. A rook may attack targets from the side, support another rook, or try to reach a horizontal row near enemy territory. Occupying the file does not guarantee an advantage, however: the opponent may contest it, exchange rooks, or control every entry square.

Control is not the same as penetration

To exploit the file, look for a square where the rook can enter safely or a target that can be pressured. If the only entry square is securely defended and there is no target, the file may have little immediate value. It also matters which side can double rooks, meaning place both rooks on the same file to add pressure.

A semi-open file contains pawns of only one color, while a closed file contains pawns of both colors. Modern usage normally keeps these definitions separate, although some older writers used open file for a file without a friendly pawn, which is now usually called semi-open.