Plain-English definitions of the chess terms that come up in Mainline, the tactics, endgame techniques, and training ideas worth knowing. Each links to a fuller entry. New here? Start with the About Mainline page.

Fork: A fork is one piece attacking two or more targets at once. If the opponent cannot save them all, you can win material.
Pin: A pin attacks a piece with the king or a more valuable piece behind it. Moving off the attack line would expose that piece; exposing the king is illegal.
Skewer: A skewer attacks a piece with another piece behind it on the same line. If the front piece moves away, you can attack the one behind it.
Discovered attack: A discovered attack happens when moving one piece opens an attack from another piece behind it. The moving piece may create a second threat.
Zwischenzug: An "in-between" move inserted before an expected recapture, changing the outcome of an exchange.
Zugzwang: A position where any move a player makes worsens their position, but they're forced to move anyway. Common in endgames.
Back-rank mate: A checkmate delivered along the back rank by a rook or queen against a king trapped behind its own pawns.
Removing the defender: Removing the defender means capturing it or making it leave what it protects. This can let you win a piece or reach a mating square.
Opposition: In direct opposition, the kings face each other with one square between. The player who does not have to move can restrict the other king’s advance.
Lucena position: The Lucena position is a winning rook endgame with a pawn on the seventh rank and its king on the promotion square. The rook blocks checks so the king can move out and the pawn can promote.
Gambit: An opening where a player sacrifices material, usually a pawn, for faster development or attacking chances (e.g. the Danish Gambit, trained on Mainline).
Repertoire: The set of openings a player chooses to use with White and Black. Mainline builds yours around the lines you actually face.
Spaced repetition: Spaced repetition means reviewing what you learned over several days. The gaps between reviews grow as you remember it.
Blunder: A move that significantly worsens your position, usually losing material or a winning advantage. Mainline turns your real blunders into drills.