I often encounter this in tactic quizzes - the answer the defender plays "in the main line" I don't even consider because I think there is something better. (I'm a strong defender, but of course I can't talk back to Stockfish, so probably my suggestion has an answer too.)

Is there any alternative to defining "the main line is the best engine move" even if that move is psychologically absurd? (I just say spite check.)

  • Specifically in tactics I believe mainline is usually the most direct attempt by defender against your combination - as such it is usually the clearest, while other moves may murk the waters a bit more. May 15 at 5:05
  • You said it, there is little chess level, human machine interaction. One is expected to swallow the engine opinion, according to some common sense.. No talking back. It talks as an expert (but does not have to, after all, completely reproducible, even its stochastics are designed, AFAIK, or artifacts of the design, if we could analyze the engine over chess (not tournament optimization jurisdiction). I thought you were asking also about the human concept of main line. Now that the tree has shown some fractal tendencies. (At least the tree is sprawling densely).
    – dbdb
    May 16 at 0:13

2 Answers 2


A term I've heard specifically in the context of puzzle-banks/tactics trainers is "critical line". The critical line would be the line that asks the most of the puzzle solver's calculation, whether it is the computer's choice or not. For complicated tactics the critical line may be a matter of opinion, for this reason many people prefer books over automated tactics trainers like Lichess or chess.com. The online puzzles may pull positions from real games, but the computer may not be able to decide which continuation is the most challenging from a human perspective.

To answer the question: The "Main Line" is usually only used in opening theory, and it refers to the most popular variation of an opening. The "Critical Line" is a variation from any position that a player must understand to justify a tactic. The "Engine Line" is the objectively best continuation in any position according to the computer. It may be unclear to humans why the engine line is better than the critical line, until several (perhaps unexpected) moves have been made. Often times the critical line and the engine line can be the same. Humans will likely play the critical line and challenge their opponent to prove that their play is sound.

  • 1
    I'm a German, i.e. most of the confusion was translation-based. (And yes, I heard "critical line" before. Only didn't recall when needed...) May 15 at 17:56
  • Each time a new variation is discovered, there comes the discomfort of what should be the mainline. If it is the first ever discovered line, that had its own name from all its depth, or if it is the most popular, or nowadays, the first PV score "line" (from also a very bushy partial tree; ok, deeper than wide, when in doubt), or the theoretical best, now that more data has been accumulated. If taking a pure name segmentation, new variations might mean a reducing ratchet over time, from the common prefix to be the only absolute mainline left. All the others lines would be the deeper ones.
    – dbdb
    May 16 at 0:18

Is there any alternative to defining "the main line is the best engine move" even if that move is psychologically absurd?

I think the obvious one is "The most played move in the database". If you want you can qualify it with "in games where both players are over 2400". This what I've always understood by the "Main Line".

  • This is an acceptable answer for "what is the main line in opening theory". I'd prefer "what is the main line in analysis", which has a wider scope. (For example, the psychological main line against a sacrifice is the capture.) May 15 at 6:32
  • To keep oranges and apples, or engine and humans in their categories, I would just say main principal variation or main PV (even if user does not ask for it, there were a bunch of candidates "PVs" for a while in the single partial tree search; asking for more than one is just a way of forcing the engine to spill more beans, as it would not in tournament constraints (unless there are PV number categories of competitions, but usually the engine plays only one move, in one game per position.
    – dbdb
    May 16 at 0:22

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