How To Read The ECO

The Encyclopaedia of Chess Openings can be thought of as a series of containers. There are really three parts to the book. The first is the ECO Code itself, which is listed in the tables of contents. Each level describes what is contained within it, until you reach the actual lines of the ECO, which is the second part of the book. Along with the annotations, a "game" is described where the mainline and each variation are separate best-practice lines. The third part of the book isn't actually in the book itself, but is the entirety of all chess openings, as they fall within the definitions.

The ECO Code

Within the tables of contents, spread out over all five volumes, is the actual ECO Code, in three separate layers.

This first layer describes A – E at their most basic. This is why it isn't even in any table of contents, but rather at the start of each volume. The R symbol stands for "various moves", and the inverted right-angle symbol stands for "without".

In natural language this reads, "A contains various moves not including 1.d4 or 1.e4, as well as anything following from 1.d4 that doesn't include 1…Nf6 or 1…d5, as well as anything following from 1.d4 Nf6 that doesn't include 2.c4, as well as anything following from 1.d4 Nf6 2.c4 that doesn't include 2…e6 or 2…g6."

In other words, the team in Belgrade led by GM Matanovic sat down some time in the 1970s and covered every opening in as methodical a way as they could, carefully stepping through every possibility.

The second layer in this example covers A0 – A9, and each letter is the first table of contents of each volume, as it defines its contents. As you can see, A4 is described in A. In turn, it describes everything covered in A40 – A49.

In natural language, this reads, "All moves following from 1.d4 that don't include 1…Nf6 or 1…f5 or 1…d5, as well as anything following from 1.d4 Nf6 that doesn't include 2.c4."

This third layer is one of the informally-named "ECO codes" that we're familiar with (as opposed to the entirety of the formally-named ECO Code). Each is found in the table of contents for its letter-number section. Again, A48 is described within A4.

In natural language this reads, "All moves following from 1.d4 Nf6 2.Nf3 g6 that don't include 3.g3."


In the fourth layer we reach the actual lines. A48 happens to have 20 of them in the 4th edition (and it varies with each release.) Every line can be written out straight across, starting with the moves listed in the header. In this case, A48/001 is written out, 1.d4 Nf6 2.Nf3 g6 3.e3 c5 4.dc5 Qa5 5.Nbd2 Bg7 6.a3 Qc5 7.b4 Qc7 8.Bb2 O-O 9.c4 d6 10.Be2 b6 11.O-O Bb7, but as you can see, it includes a host of footnotes, and those make up the variations of the line.

As you also might notice, the symbols for checking or capturing are both missing. While it is absolutely essential that you let software fill these in, and never try to figure it out in your head (when writing out the movelist), one good rule of thumb is that when there are two adjacent pawn designations, such as with 4.dc5, then a capture is taking place, as it can't mean anything else.

This is the footnote for 1.d4 Nf6 2.Nf3 g6 3.e3, meaning that it presents alternatives to 3.e3. To enter these in, I personally need either ChessBase or HIARCS Explorer. Having done so, this is the result:

1. d4 Nf6 2. Nf3 g6 3. e3 (3. b4 Bg7 4. Bb2 O-O 5. e3 (5. Nbd2 d6 6. e3 c5 7. dxc5 dxc5 $11 {Alexandria – Chiburdanidze, Tbilisi (m/2) 1977 – 23/98}) 5… b6 6. Nbd2 c5 7. dxc5 bxc5 8. b5 d5 9. c4 Nbd7 10. Be2 a6 11. a4 axb5 12. axb5 Bb7 $11 {Polugaevsky – Nunn, Hastings 1992/93}) 3… c5 4. dxc5 Qa5+ 5. Nbd2 Bg7 6. a3 Qxc5 7. b4 Qc7 8. Bb2 O-O 9. c4 d6 10. Be2 b6 11. O-O Bb7 $11 *

You might notice that there is lots of room left for the rest of the footnotes. One line of the ECO is roughly the length of a normal PGN.

It isn't an actual game, however, as it too is a container for the suggested lines related to that ECO code. If you separate out the variations described in that one footnote, you get the following lines:

1.d4 Nf6 2.Nf3 g6 3.e3 c5 4.dxc5 Qa5+ 5.Nbd2 Bg7 6.a3 Qxc5 7.b4 Qc7 8. Bb2 O-O 9.c4 d6 10.Be2 b6 11.O-O Bb7 [The original line.]

1. d4 Nf6 2. Nf3 g6 3. b4 Bg7 4. Bb2 O-O 5. e3 b6 6. Nbd2 c5 7. dxc5 bxc5 8. b5 d5 9. c4 Nbd7 10. Be2 a6 11. a4 axb5 12. axb5 Bb7 [The first variation in the footnote.]

1.d4 Nf6 2.Nf3 g6 3.b4 Bg7 4.Bb2 0-0 5.Nbd2 d6 6.e3 c5 7.dxc5 dxc5 [The second variation in the footnote.]

After entering in all variations and separating them out, that one "variation" could contain 20 or so actual best-practice lines.

As A List Of Openings

Finally, the part that most of us associate with the ECO — and which many of us start out looking for. The named variations. These are the actual openings known in the world of chess but not actually named anywhere within the ECO itself. The ECO Code, as set out in the tables of contents, decides which opening falls within which code, but nothing more. Each line provided within the book is the current theory about the best way to play that particular opening strategy.

To finish this example, these are the conventional openings covered in A48:

King's Indian: East Indian Defence
1. d4 Nf6 2. Nf3 g6

King's Indian: Torre Attack
1. d4 Nf6 2. Nf3 g6 3. Bg5

King's Indian: London System
1. d4 Nf6 2. Nf3 g6 3. Bf4


ECO Code Page At 365Chess
Computer-Readable Definitions At MorphyChess
Encyclopedia Section Of Chess Informant Store

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