The opening of a chess game is the initial stage of the game. Both players develop their pieces and try to prepare their middle game. Many openings have standard names, such as Sicilian Defense or Russian Game. Opening positions are defined by a given position on the board. A game transposes to a different opening, if it reaches a position which is normally reached by a different move order. In professional chess, opening transpositions are used to avoid certain lines, trick your opponent or force the opponent to play an unfamiliar position.
For example, there are two move orders to reach the Queen's Gambit:
| Move | Opening Name |
|---|---|
| 1. d4 | Queen's Pawn Game |
| 1. ... d5 | Closed Game |
| 2. c4 | Queen's Gambit |
| Move | Opening Name |
|---|---|
| 1. c4 | English Opening |
| 1. ... d5 | Anglo-Scandinavian Defense |
| 2. d4 | Queen's Gambit |
The first order is considered solid and played very often. The second is considered unsound and therefore (almost) not played by pro players. If we analyze many chess games, we can build a graph were each node is a known opening position and edges are transpositions between openings (or opening variants).
I analyzed 339,024 games between pro players from Lichess from April 2022. The opening names are the same as used in the opening explorer of Lichess. I parsed the games with python-chess and extracted the epd1 after each of the first 30 half moves2. I then checked if this epd has a name. If the current position has a name that is different from the previous position, I added a transposition between the old and the new position. If it has no name, I kept the name of the last named position as current opening line.
The resulting graph is visualized with Gephi. The layout algorithm works so that connected nodes attract and not connected nodes repell each other. After a while we get a stable configuration where groups of strongly connected nodes emerge.
To get the colors I used the modularity algorithm. Modularity is a measure of how well connected nodes in a graph are. So we can find groups of nodes that are better connected to each other than to the rest of the graph. Each color represents such a group. The size of a node is determinded by the number it has been reached. The thickness of an edge represents the number this specific transposition happend.
First we see the entire graph. We can clearly see a few very distinct groups and some groups that are very similar and not distinguishable. But there is also a lot of chaos. There are 2479 nodes total, connected by 6503 edges. Each color represents one or more opening families:
| Color | Openings |
|---|---|
| Orange, Yellow | Queen's Pawn: Queen's Gambit, Other Queen's Pawn Games |
| Green | Indian, Grünfeld, Benoni Defense(s) |
| Red | Zukertort, English, Réti Opening |
| Blue | King's Pawn: Pirc, Scandinavian, French, Caro-Kann, Alekhine |
| Purple | King's Pawn: Scotch, Italian, Russian, King's Gambit, Ruy Lopez |
| Turquoise | Scicilian Defense |
| Grey | Noise, unregular openings, weird gambits |
If we filter seldom (< 300 times) played variations out, we get a more clear graph:
Now we look at each group individually. I filtered nodes with very low number of occurrences out so that we can see some of the variation names. Each subgraph again is run through the force atlas algorithm with Gephi.
Using graph theory, we confirmed that strongly connected nodes in the graph are indeed variations of the same openings. There is e clear distinction between Queen's Pawn Games on the left and King's Pawn Games on the right. It's said that transpositions are more frequently and important in Queen's Pawn Games than in King's Pawn Games. Using the graph we can confirm this. In particular the English and the Zukertort Opening often transpose into other lines of the Queen's Pawn Game or Indian Defense.
There are ~3300 named opening positions in the Lichess Database. I was surprised to see 2500 of them in only one snapshot of one month.
I used the opening data from Lichess. Each named opening (variant) has a name and a epd 1.
The games are downloaded from the Lichess Elite Database. I used the file from April 2022. The file is in pgn format. PGN files contain one or more chess games. Each game has a header with metadata and the list of the moves in the game in Short Algebraic Notation. All players had a rating > 2400.
I changed the name of the position after 1. d4 d5 from Queen's Pawn Game to Closed Game to get some differentiation to other 1. d4 lines.