French Classical:Idea behind 12.Kb1?

I was going through the main line of this opening,and i have a doubt.What is the idea behind white's 12th move?What does it achieve?

With myself not being by a French book right now, I can tell you a few ideas as to why it's done, but if you are looking for specific deep lines of analysis, you'll need to look elsewhere.

For starters, of course with the pawn structure, White will attack Kingside, Black Queenside.  The fact that the c-file is already semi-open with a Rook ready to get there in 2 moves and the Queen already on the semi-open c-file, White would like to get out of dodge of that.

Secondly, If Black is ever able to place pressure on b2 or a2, the King is additional coverage for a2, allowing the Knight to eventually be freed from duties at c3, and also, White may need c1 for a heavy piece at some point.  Whether it be the Queen to cover b2, or the Rook to advance c4 at a time when a ...d4 response is bad.

Not so sure how fond I am of 11...Nb6 without advancing the Queenside pawns, but then again, even when I did play the French as Black, I didn't play the classical much.  Played the Winawer in my earlier days, and the MacCutcheon in the latter days of playing the French, with the Classical having a brief stint around 2003.

Just as when you 0-0 a common move is Re1 when you 0-0-0 a common move is Kb1.  This is in all openings (and Kb8 with black 0-0-0 of course).  The king is less accessible (moves off the more centralized c1-h6 diagonal and c file) so it's a preparatory move that allows the white player to undertake other actions... it's the same as the advice to not attack before you develop.

In that specific position because black will be attacking the queenside esp on the c file.  But also if white just wanted to seek play on the kingside it's a good preparatory move before opening the position.

Better be careful about stereotyping that Kb1 move.  It's a complete waste of time in many cases, especially in the Najdorf, where every tempo matters.

Really?  I don't play mainline Sicilian stuff, so I don't know.

But chess is full of basic advice that has a thousand exceptions.  King to the knight file is a common preparatory move at some point after 0-0-0 in many different openings.  I'm sure there are many exceptions, but they are... well... exceptions :p