How much theory does the book actually cover and how far in terms of opening knowledge and building my opening repertoire will memorising every line of the book get me?
The best uses for out of date opening books are -
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how far in terms of opening knowledge and building my opening repertoire will memorising every line of the book get me?
You will be an expert in historical opening lines, but you will understand nothing.
MCO, latest edition 2008 as far as I know, is mostly composed of chess lines with minimal notation. Each line usually finishes with an evaluation like "=", "+=", "-+", etc.
I have the 11th edition from somewhere round about 1970. The last time I looked up a line in it was sometime in the 1970's. The main reason I still have it is that it is where I keep my signed scoresheet of my 1972 draw with Victor Kortchnoi - in a simul, obviously.
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This book is a reference manual. Each page shows a diagram position from 2 to 10 moves deep. Beneath the diagram columns of chess notation denote how different games have proceeded from that position. The next page will usually contain the many footnotes referenced from the columns. These include source games or mentions of interesting alternatives.
Is it interesting? Well, it is a little bit. Maybe once a year it's even useful! For the most part, though, the information within this book is available via online tools that are both free and more complete.
Why consult the MCO about ideas in the Berlin when I can watch Yasser explain it to me on YouTube? If I'm curious about a particular line then I can plug it into an analysis board faster than I can even find the MCO on the shelf.
There's a reason why the MCO stopped in 2008. It is only useful to those who are unable to use modern home computers.
You aren't going to learn much that you couldn't learn from a database. MCO has very little explanation.
I will say though that it is useful to quickly run through an opening and get a feel for the types of typical positions that come up. Also, sometimes the positional evaluations might be interesting especially if they contradict the engines and databases.
Memorizing lines won't get you anywhere unless you're playing someone who tried to memorize the same but did a worse job at it. I can honestly say I have never once tried to memorize a line in the entire time I'm played chess. I do my opening analysis one time and usually remember most of it because I understand why I'm making the moves. If it comes up in a game and I don't remember it, I'll go over the analysis again. Usually that's enough.
For normal players (ie below GM) most openings aren't going to be more than about 8-10 moves deep. The first few moves you won't have to memorize because you play them all the time. So really, learning a line is learning about 4-6 moves which isn't hard if you understand why you're making those moves.
MCO is an excellent source of information. In fact, opening encyclopedias actuality are not related to the existence of chess engines or to the latest that has been played. Rather, the variants selected by the encyclopedia are important because the players who selected them are people with extensive experience in the field.
The time that has elapsed since they were selected does not mean that they are no longer current. The existence of the Spanish opening, for example, or other similar ones for centuries, testify to the solidity of these openings.
With the passage of time, players have found new ways of dealing with openings and new channels to lead the game towards the forms of play that are convenient for them, but thats not for everyone.
I provide a simple example. MCO helps research lines with potential for postal chess that resist attack from chess engines. I do not want to go into details of how I achieve it, but it is a laborious effort in which I compare the evaluations that the Masters recorded, against the result of recent games from the final positions of those lines in face-to-face and correspondence games.