When they deviate from book, that usually means that they played a sub-optimal move. You should be glad, because that means they just handed you an advantage.
What if the opponent doesn't cooperate?

If you understand the opening, and have learned all the lines, and your opponent has played something that is not in the books, then he has done something wrong. In other words, he has made a mistake. Take your time, find the mistake, and exploit it. There is also the possibility that he has prepared something, like I prepared here:

If you understand the opening, and have learned all the lines, and your opponent has played something that is not in the books, then he has done something wrong. In other words, he has made a mistake. Take your time, find the mistake, and exploit it. There is also the possibility that he has prepared something, like I prepared here:
WRONG! New ideas are found all the time. Just because some book on the Najdorf only mentions 2 moves for Black on move 23 doesn't mean that there can't be a better idea. One book! One author! One person's ideas! It ain't like every author of every book has a 64-bit processor for a brain. New ideas are found all the time. Why do you think John Nunn's 2 books on the Najdorf from 1996 and 1997 are considered outdated? Because new ideas are found all the time that changes the assessment completely.
So just because your opponent plays a move that isn't printed and bound in that lump of killed trees that you are holding onto doesn't mean the move is bad or inferior!
Case in point - There is a 552 page book covering 1.e4 c5 2.a3. Most anti-Sicilian books written for Black don't even mention 2.a3. Does that make 2.a3 a bad move and those 552 pages written on it are just utter junk?

lolurspammed wrote:
Memorizing will only get you so far. Your opponent can play g4 on move one. Where's your prep now?!?!
Case in point!

Like I have said before...
you studied all that theory on the Berlin Wall and it's endgame for black... thats cute...
2. f4

Case in point - There is a 552 page book covering 1.e4 c5 2.a3. Most anti-Sicilian books written for Black don't even mention 2.a3. Does that make 2.a3 a bad move and those 552 pages written on it are just utter junk?
i have said book and applied some lines with success (as prepared lines) in some games, it's pretty cool. :D
the point is that every move played ("theory" or not) has some kind of idea behind it, and it's up to you to find the best counters to these ideas.
these counters might or might not be covered by theory. if they aren't, before ditching your opponent's move as bad, try to find out what he's trying to do and why it shouldn't work as he is intending... if you can't, that's bad luck.

Yeah, maybe that's what the OP didn't realize... that opening study also involves what to do if other moves are played, not just a single memorized sequence.

Case in point - There is a 552 page book covering 1.e4 c5 2.a3. Most anti-Sicilian books written for Black don't even mention 2.a3. Does that make 2.a3 a bad move and those 552 pages written on it are just utter junk?
i have said book and applied some lines with success (as prepared lines) in some games, it's pretty cool. :D
the point is that every move played ("theory" or not) has some kind of idea behind it, and it's up to you to find the best counters to these ideas.
these counters might or might not be covered by theory. if they aren't, before ditching your opponent's move as bad, try to find out what he's trying to do and why it shouldn't work as he is intending... if you can't, that's bad luck.
Inyustisia - Here's one of my favorites where I played 2.a3 and destroyed my opponent back in the Spring:
http://www.charlottechess.com/games2/1269.htm

it's an improved wing gambit. compare the move orders:
1 e4 c5 2 b4 cxb4 3 a3
1 e4 c5 2 a3 .. 3 b4
black has a lot of options in move 2, but out of the replies that allow a plausible 3 b4 he can't play the critical continuations ..d5! and ..bxa3 compared to the original wing gambit move order. of course, though, 2..Nf6 and 2..d5 are playable replies too (though arguably worse than against the alapin), and 2..g6 is also good.

yes of course 2..ab 3 a3 d5. i don't really believe in like these lines myself though... my opinion is that there certainly is compensation, but it's not quite enough if your opponent can prep.

Anything is playable against the wing gambit...it's it's not as critical and dangerous as the Morra.

Anything is playable against the wing gambit...it's it's not as critical and dangerous as the Morra.
The Wing Gambit Proper, yes I agree, especially 1.e4 c5 2.b4 cxb4 3.a3 d5!, which is actually good for Black. But the deferred wing gambit - I beg to differ with you about how dangerous it is.
Case in point (You go and tell me 2.a3 isn't dangerous after this!):

Anything is playable against the wing gambit...it's it's not as critical and dangerous as the Morra.
The Wing Gambit Proper, yes I agree, especially 1.e4 c5 2.b4 cxb4 3.a3 d5!, which is actually good for Black. But the deferred wing gambit - I beg to differ with you about how dangerous it is.
Case in point (You go and tell me 2.a3 isn't dangerous after this!):
What do you play against 2...g6! though?

Anything is playable against the wing gambit...it's it's not as critical and dangerous as the Morra.
The Wing Gambit Proper, yes I agree, especially 1.e4 c5 2.b4 cxb4 3.a3 d5!, which is actually good for Black. But the deferred wing gambit - I beg to differ with you about how dangerous it is.
Case in point (You go and tell me 2.a3 isn't dangerous after this!):
What do you play against 2...g6! though?
White's fine after 2...g6:
If you don't know what to do when your opponent goes out of "book", then what you did is utterly useless and you know absolutely nothing. Anybody can parrot moves. That's not what studying openings is about. WHY is each move made? If you don't understand the underlying reasons behind each move, you've done nothing useful what-so-ever!
Keep in mind, when I say that you don't know what to do, I don't mean that you can't just parrot out a move in a split second. What I mean is that you have no clue how to follow up strategically. There is a reason for the book move. If you know that Black is supposed to play 14...Nc6 because it covers up a weakness, and failure by him to do so means he has an exposed weakness on say, b7, and your opponent plays 14...h6, then you ought to know that the plan is to attack b7, and spend the time figuring out how to do so. If all you know is "Black was supposed to play 14...Nc6 so that I can play 15.a3", then you don't know sh*t and are completely wasting your time.