Principal variation: definition and usage in chess

Principal variation

Definition

The principal variation (PV) is the best line of play from a given position, assuming optimal moves by both sides. In human analysis it is the “main line” you believe is most accurate; in computer chess it is the engine’s current best sequence, continuously updated as search deepens. The PV shows not just the best move, but the follow-up moves that justify it.

How it is used in chess

• Human analysis: Annotators and coaches present a PV to communicate their main recommendation in an opening, middlegame plan, or endgame line: “PV: 1...c5 2 dxc5 Qxd1+ 3 Kxd1 Bxc5, with an equal endgame.”
• Engines and GUIs: Chess engines output a PV with a score and depth. Example readout: “Depth 22 | +0.86 | pv: 1...c5 2 dxc5 Qxd1+ 3 Kxd1 Bxc5 4 Ke2 O-O 5 Be3.” The PV can change as depth increases.
• MultiPV mode: Engines can show several top candidates (MultiPV). PV #1 is the current best line; PV #2, #3, etc. are strong alternatives.
• Preparation and training: Players build opening repertoires around trusted PVs and use them to memorize critical forcing sequences and typical middlegame ideas.

Strategic and historical significance

Strategically, a PV is the spine of your calculation: it tells you “what happens if both sides play best.” It helps compare candidate moves by projecting outcomes move-by-move. Historically, the term comes from computer chess (1960s–70s), where alpha–beta search would maintain the currently best (principal) line, and specialized methods like Principal Variation Search (PVS) optimized searching moves on that line. Today, PVs bridge human and machine analysis—annotators present a human-readable PV, and engines justify evaluations with theirs.

Examples

• Opening PV (Closed Ruy Lopez main line idea):

  One commonly taught PV to reach a solid Closed Ruy position is:

  PV: 1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O Be7 6. Re1 b5 7. Bb3 d6 8. c3 O-O 9. h3

  Example viewer:

  
  

  Why it matters: This PV sets up the classic maneuvering battle with d2–d4 ideas, Re1, Nbd2–f1–g3, and pressure on the e5 pawn.

• Tactical PV (Légal’s Mate motif):

  After 1. e4 e5 2. Nf3 Nc6 3. Bc4 d6 4. Nc3 Bg4?!, White has 5. Nxe5! If Black greedily plays 5...Bxd1?? then the forcing PV is 6. Bxf7+ Ke7 7. Nd5#—a model mating net exploiting pins and a centralized knight.

  
  

• Engine PV snippets (how to read them):
  • Depth 23 | +0.60 | pv: 1. Re1 Re8 2. h3 h6 3. Be3 Bf8 4. Qc2 c6 5. Rad1
    - “+0.60” means White is better by roughly 0.6 of a pawn if both sides follow the PV.
  • Depth 31 | #5 | pv: 1...Qh4+ 2. g3 Qe4 3. Kf2 Bc5+ 4. Be3 Qxe3+ 5. Kg2
    - “#5” indicates a forced mate in 5 plies for the side to move.
  • MultiPV=3:
    1) +0.35 pv: 1. ... c5 2. dxc5 Qxd1+ 3. Kxd1 Bxc5
    2) +0.20 pv: 1. ... O-O 2. c4 e5
    3) +0.10 pv: 1. ... a5 2. a4 Be7
    - The engine prefers line 1), but shows viable alternatives.

Practical tips for working with PVs

• Check stability: If the PV keeps changing every few depths, the position is complex—don’t trust a single shallow PV.
• Understand evaluation perspective: Most engines give scores from the side to move; “+” favors White, “−” favors Black.
• Prioritize forcing lines: In tactics and endgames, the PV is often a series of checks, captures, and threats; verify every branch.
• Use MultiPV to avoid tunnel vision: A narrow PV can miss resources; compare top candidates to understand plans.
• Translate PVs into ideas: Don’t just memorize moves—extract themes (pawn breaks, piece routes, king safety) so you can improvise over the board.

Interesting facts and anecdotes

• Computer-chess origin: Engines maintain a principal line as they search; specialized methods like Principal Variation Search (PVS) try likely best moves first, saving time if those moves hold up.
• Kasparov vs. Deep Blue, 1997: Much of the commentary revolved around “what is the machine’s PV here?”—a public showcase of how PVs explain engine choices.
• Tablebases and PVs: In solved endgames, the PV is exact and unique up to move-order nuances, often revealing surprising “only moves.”
• Annotators’ shorthand: In books and databases, “main line” and “principal variation” are used almost interchangeably; sub-variations are indented under the PV.