Openings › How it works
How the numbers are made
- Source: Lichess open database, file for 2026-09, released under CC0 1.0. Chess Opening Scores is independent and not affiliated with or endorsed by Lichess.
- Sample: 90 slices spread evenly through the month's file, covering games from 1 Sep 2026 – 30 Sep 2026 (30 distinct days). Fixed byte offsets spread evenly across the monthly file: offset_k = (k + 0.5) / slices * file_size. Each slice is decoded from its first complete compressed block, so the sample covers many moments across the month. Reproducible: same month + same settings = same games. Sample size by data month: from 2026-08, 30 slices; from 2026-09, 90 slices. September 2026 was re-sampled with 90 slices when the sample size was raised on 2026-10-03; these numbers replace the earlier 30-slice version.
- Games read vs. used: 483,833 games read; 456,087 used after filters (rated bullet, blitz, rapid, classical games; players within 200 rating points; excluded terminations: Abandoned, Rules infraction). Excluded: termination 1,742, speed 5,221, result 1, rating 20,782, unreadable moves 0. Games whose first moves cannot be read are counted as read but excluded from "used", from rating-level totals and from every percentage.
- Time controls: as labelled by Lichess (bullet, blitz, rapid, classical). Rating-level and time-control tables are separate summaries; neither is split by the other.
- Lichess ratings: ratings come from Lichess and are not interchangeable with Chess.com or FIDE ratings.
- Data period vs. page updated: the data period is when the sampled games were played (1 Sep 2026 – 30 Sep 2026). "Page updated" is the date of the last meaningful data or content change (3 Oct 2026).
- Reproducibility: file https://database.lichess.org/standard/lichess_db_standard_rated_2026-09.pgn.zst (last modified Sat, 03 Oct 2026 05:06:59 GMT), 90 slices of 4,194,304 bytes; offset_k = int((k + 0.5) / slices * file_size).
- Rating level: average rating of both players. Levels have no gaps: each covers from its lower limit up to, but not including, the next level's lower limit (e.g., an average of 1199.5 is "Under 1200").
- Matching openings: a game counts for an opening if it reached the same position after the same number of moves — including different move orders. Positions reached after a different number of moves are not counted.
- Minimum sizes: 300 games to show a rating level, 150 games to show a reply. A page is published only with at least 3 rating levels shown and 2,000 games in total.
- Margins of error (±): approximate 95% margins based only on the number of games. They do not cover sampling bias (games come from specific moments of the month) or related games (the same players play many games), so real uncertainty is larger. Small differences may be chance, and these comparisons alone can't show that one reply is reliably better.
- What the numbers mean: observed results for the selected games. They don't prove that an opening causes wins — stronger or weaker players may simply prefer certain openings.
- Text: every sentence on the opening pages is generated by code from these numbers.
- Downloads (CSV, CC0): each opening page links a file with the same numbers as its tables. Columns: summary (rating level, time control or reply), level or move, games, White win / draw / Black win %, score %, approximate ±95% margin, and whose score it is ("score_for"). Groups with too few games keep their count but have empty percentages, as on the page.