There is an old tale that goes something like this: The great Botvinnik (the pioneer of modern opening preparation) was once consulted about opening advice by one of his pupils. "Look at Fischer," the master began, "Fischer plays the same systems over and over again. He knows all the nuances quite thoroughly and has good results with them." "But," the student retorted, "Isn't it better to know more than one system well, to be able to surprise your opponent?" Now Botvinnik turned to Smyslov, who was seated besides him, and said, "Look at this guy. He knows and plays a lot of systems. He is a true artist. But he is a good analyst, not really a player. You see, this is the difference between him and Fischer; better to play one system and know it well than play many and know it haphazardly."
Botvinnik was expressing an ancient truth, which was discovered long before modern chess and had nothing to do with the royal game but rather with human nature in general, namely the idea of "jack of all trades, master of none!"
Indeed, in our modern age especially, it is important to have a well-thought out and developed repertoire. In fact, a psychological study was done in the last twenty years in which particpants, ranging from beginner to Grandmaster, were given chess problems to solve. When confronted with a position that had arisen out of the participants repertoire, the researchers found that, on average, the subjects performed one "level" above their strength. Thus, a master who played the Sicilian Scheveningen, when given problems coming out from this opening, performed just as well as the Grandmasters on average (who did not specialize in this particular opening). A master could play like a Grandmaster when in his specific system!
It seems incredible but it is really true (I should know, I did a whole term paper about this in high school). I have a friend who plays the same systems over and over again - but he knows them like an encyclopedia. And he has admitted to me that he mainly exclusively works on this part of the game, always trying to treak and improve his repertory (and I have seen it myself!) Oh, and by the way, he is already a Grandmaster and one of the strongest American-born players ever - you may know him as a famous Chess.com personality.
Ok, enough jibber-jabber about the importance of building a narrow repertoire; I suspect you may be getting slightly anxious already. The real question is now, "how do I go about building my repertoire?" There is a two-part answer to this one-part question. The first part adresses preciesly how to, or, in other words, the method of going about building a repertoire. Nowadays every chess player has access to a database containing millions of games, from the olden times to games played in the last year! Incredible! Just imagine, fifty years ago, when modern preparation was in its infancy, Soviet players would keep index cards (or notebooks) with their analysis of the variations! Just think, hundreds of notecards filled with moves and moves! Well, I guess we should say, hooray for modern technology!
But all the access to the databases won't help without the aiding of a good coach, or, if you don't have one, yourself (but be willing to put in the effort!) I'd like to quote Mickey Adams here when he says that "all openings are playable but they are not all playable for you." What he meant by that is the fact that humans are all different by nature; we have different personalities, different tendencies, different predilections, different likes and dislikes; in short, we have different strengths and weaknesses, and what may suit Ali may not be right for Baba (and vice versa). Karpov would never play a Najdorf consistently; Kasparov would never opt for the Caro-Kann (although he played this in his youth, then was told by Geller to stop, and, as they say, the rest is history).
Obviously, I would like to take you through the process of building a repertoire using a real example - but I cannot elaborate on every variation, else this article would be way too long. After some thought I chose the Scheveningen variation of the Sicilian (in positional rather than combinative style). I think you will like my choice. I had the pleasure of talking with GM De Firmian (the writer of MCO) a few years back and he mentioned that in most openings one could get by with a basic knowledge and common sense. However, the Sicilian is one opening which one should know thrououghly and have a good experience, a good feel, for.
As I mentioned, we should all have access to databases nowadays, and I would encourage you (for any opening) to compile not more than ten games (preferably with notes) study them thoroughly, and practice your variant in a few casual games to get them in your bloodstream. What I would like to do is to start this process, to analyze with you a "root" game of the Sicilian Schevingen - perhaps the most famous Scheveningen game in the last fifty years.
The following game was the final game in the 1985 Karpov-Kasparov match. Karpov held the title for ten years. He was White, and if he had won this game, he would have retained his title further, and chess history might have been completely different. Karpov had an advantage, and at the critical moment, became timid and erred. Let's get to the game. Before we go though, I would like to remind you that many of us are busy, and we have no time to learn the main lines. Many sidelines are perfectly playable, but the stronger the opponent, the less of a chance one may have using a sideline. So pick your priorities and enjoy!
A complicated game, a valiant fight! And a lot of unanswered questions. A more detailed study of this game and a few other modern games will get one ready to play this variation in time.
So, if you are so inclined, get to work! Otherwise, relax, learn some side-opening lines; you'll be fine too - just, if you are playing Kasparov, you may be in trouble!