Lichess Elite Opening Books (Polyglot)

[Tobias]

Built from 3.8M Lichess games (players ≥2400 vs ≥2200), this pack contains five themed opening books plus a combined Allbook. Goal: practical, Lichess-style play, broad choice, no junk. All moves are equally weighted, so the engine doesn’t bias for popularity and stays more diverse.

Books (games used):

• lc-open.bin – 1.e4 e5 → 324,769

• lc-semiopen.bin – Sicilian/French/Caro-Kann … → 1,342,110

• lc-closed.bin – 1.d4 d5 → 306,022

• lc-indian.bin – 1.d4 Nf6 (King’s Indian, Nimzo/… ) → 283,735

• lc-flank.bin – 1.c4 / 1.Nf3 / 1.g3 … → 579,000

lc-allbooks.bin merges all five; duplicate entries are averaged, preserving the flat weighting.

What sets them apart

• Flat weighting: several sound candidates per node.

• Quality filters: only moves with sufficient games/share; minimum continuation depth; gentle transposition smoothing to include near-threshold candidates without noise.

• Practical depth: typically around 20–22 ply, depending on family.

• Safety checks: Polyglot verification and backward check (legality, parent/child hashes).

Usage

• Format: Polyglot .bin (Zobrist-sorted).

• Works with PicoChess, SCID, Lucas Chess, and many UCI GUIs.

• Load the book and enable it—moves are chosen with equal probability until out of book.

Thanks: PGN data courtesy of https://database.nikonoel.fr/

[Johan Sjöblom]

Thanks Tobias.

I tested the lc-allbooks.bin on the Pico v4 beta, but it started making very strange opening moves as black like 1.e4 g5 and 1.e4 e5 2. Nf3 f6.

But I was testing the new LC0 nets at the same time (mean gyal)... I need to test more... but I was remembering that in order to use the opening books on picochess one needed to run some program to make it fit picochess... And at the moment I dont remember which program it was. So maybe the .bin file needs to be manipulated with something before used in picochess?

- Johan

[Tobias]

Hello Johan,

Thanks for your feedback! 

You’re right—there’s an issue with the allbooks build: 

allbooks.bin was generated from the finished individual .bin books rather than directly from the PGN files. 

The individual .bin books themselves are fine.

I’m currently working on V2 with a larger dataset, more recent games, and improved filters. 

I’ll upload them here over the next few days, and the allbooks build will be error-free as well.

Best regards

Tobias

[Johan Sjöblom]

Nice, looking forward to testing the opening books!