A principal variation, or PV, is the sequence of moves the engine expects to be played from a position, best play by both sides. It is the evidence behind the evaluation: the score says who is better, and the principal variation shows the line that justifies it.
An evaluation on its own is an assertion. The principal variation is the argument attached to it. Learning to read one turns an engine from an oracle into a teacher.
What the line shows
The principal variation is the best sequence the engine found for both sides. It is not a prediction that the game will follow those moves, because both players have to cooperate for that to happen. It is a demonstration of the engine's reasoning.
1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O Be7Each move in the line is the engine's choice for the side to move at that point. If you reach the end of the line, the engine will usually extend it, because its expectation simply continues further than the depth it reported.
Why the line matters more than the score
A score of plus two tells you the engine likes White's position. The line tells you why: which piece is coming to which square, which pawn weakness is being targeted, whether a king is about to become exposed. If you cannot see the idea in the line, you have learned nothing from the number.
Multiple lines and MultiPV
Engines can be asked for several best lines rather than one. This is called MultiPV, for multiple principal variations, and it is usually shown as a numbered list of alternatives, each with its own score.
| MultiPV | Rows returned | Useful for |
|---|---|---|
| 1 | The single best line | Fastest analysis and the clearest recommendation |
| 2 to 3 | The best few lines | Seeing whether there is more than one good move |
| More | Many alternatives | Study and opening research rather than practical play |
Reading a line critically
- Check whether the first move is actually playable for you. A line that begins with a move you would never find is not a plan.
- Look for the moment the line stops being natural. The engine's idea often sits where a human would have played differently.
- Compare two lines when they lead to similar scores. That usually means the position is flexible and the choice is a matter of taste, which is useful to know before agonising over it.
- Watch the score at the end of the line. If it is drifting as depth rises, the line is provisional.
Variations in a PGN
The word variation also appears in PGN, where it means an alternative line recorded in parentheses. The idea is the same: a branch that was considered rather than played. A game record can hold a main line plus any number of variations, which is how an annotated game keeps its analysis next to the moves.