Logic behind ECO codes?

I am studying old chess games and want to be able to select certain openings. Is there any logic behind the ECO codes, or do you simply have to learn them all by heart?

Yes and no. The first letters are assigned based on certain criteria, after that it's pretty much memorizing the range of codes that cover a particular opening.

Reference http://en.wikipedia.org/wiki/ECO_codes

More logical schemes here Opening Names not ECO-friendly.

The logic generally is that the further you go in a certain letter, the more specific and 'main line' the opening is.

For example, the Sicilian begins at B20 or 21 and you'd find moves such as

e4-c5
a3 or b4 or Na3

in the first chapter and the Najdorf, arguably the absolute main line of the Sicilian, between B90 and B99.

You'd find opening moves such as 1. c3 in A00 (if it is included at all :) ). Of course, it would be somewhat silly to state that 1. d4 is more mainline than 1. e4, or 1. d4-Nf6 is more mainline than 1. d4-d5, so it is best if we don't think about why the five volumes are ordered in the way they are ordered. A is for 'flank openings' B and C for e4, D and E for d4, and that's the way it is.

Of course, what is absolute main line is not set in stone. For example I think Black doesn't like C99 (mainline Chigorin defence to the Ruy Lopez) very much nowadays and instead prefers the Breyer variation (C95?).

There's a book that cross references the ECO codes to the traditional `names'. Sorry I don't have it handy, but it's exactly what you could use. Also, if you can afford them, try to purchase a traditional opening book, such as NCO, Nunns Chess Openings, and also at least the one volume eco book by Informant. The complete set of Encyclopedias are awesome, but a little pricy.

I have downloaded the TWICfiles app, which I find quite useful at the moment. Perhaps I'll look into one of the books later on. Thx.

You don't have to remember or memorize all 500 ECO codes. Just remember all you play as white and black. If you play Najdorf it's B90-B99. If you are White play e4 regularly Scotch C45. Petroff C43 and C44, etc. Your repertoire is most likely less than 50 ECO codes.

Here is a list
https://en.wikipedia.org/wiki/List_of_chess_openings

Actually, for most, it is usually above 50 if you combine what they play as White as well.

For example, if you play 1.d4 and 2.c4, and you play main lines, not side lines, you have to deal with:

D07-D09 (Chigorin and Albin)

D15 ( a6-Slav lines)

D18-D19 (6.e3 Slav lines)

D23-D29 (Black's responses in the QGA to 3.Nf3 and 4.e3)

D33-D34 (Tarrasch Defense, White fianchettos the LSB)

D45-D49 (Meran)

D55-D59 (Lasker and Tartakower)

D60-D69 (Orthodox)

Tack on what you play against the Old Indian. Benoni/Benko, Dutch, Grunfeld, 1...Nf6/2...e6, and the Kings Indian and you will be over 50 just with White.

For just Black, you can probably keep it below 50.  Let's see, for me, it is:

A00-A04, A08, A10, A80, A90, C00-C02, C08-C19 - 22 ECO codes.

With White - I could just play A00 (1.b4), but I also play 1.e4 (Closed Sicilian, Slow Italian, 3.Nc3 vs French, Both exchange and fantasy Caro, Saemisch King's Indian (3.f3 g6 4.c4 Bg7 5.Nc3 against the Pirc) and Old Indian (3.f3 e5 4.d5 vs Pirc)) along with d4 ( Trompowsky, Queen's Gambit, 2.Bg5 anti-Dutch) and 1.Nf3 (Exchange KID, Anti-Grunfeld, Catalan, English, etc)