Move order in the Richter-Rauzer

So after

the mainline is apparently 7...a6, and then after 8. 0-0-0 black goes 8...Bd7

Why this particluar move order, why not Be7 and short castle right away?

Bump. I still don't quite get it.

Be7 is also a known main line.

Without a book in front of me, just offhand, ...

If 7. .... Be7 8. OOO  OO then you need to decide what to do after 9. Bxf6 [i.e. gxf6 and try to survive the coming attack or Bxf6 then 10. Nxc6 bxc6 11. Qxd6 winning a pawn. It may be OK but you'd need to check it. ]

The a6 then Bd7 move order leads to the ''Kozul suicide variation''. It's a sharp line. The idea of it is that after Bxf6 gxf6 black often wants to play the dark square bishop to h6. So black saves tempo(s) getting pieces on the right squares. It is just one way to defend the Richter Rauzer as black.

1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 Nc6 6.Bg5 e6 7.Qd2: 7...a6 8.O-O-O Bd7 is the "modern" (e.g. 1960s) way. 7...Be7 8.O-O-O O-O is the older (e.g. 1950s) way. Either one is okay for black, both have a lot of theory, both are popular choices today. The suggestion by SwimmerBill 9.Bxf6 Bxf6 10.Nxc6 bxc6 11.Qxd6 Qb6 is not so good for white. Engine at low depth says 0.00 but let it run, actually white has some problems with the b2 square and black already has the initiative. The oldest move for white is 9.f4, next oldest is 9.Nb3 (both of these were recommended in different editions of Nunn's Beating the Sicilian), and youngest is 9.f3 (white intends Bg5-e3 and g2-g4 like in the English attack).

In order to understand why some moves are popular (we can call it "new theory"), we need to know what moves used to be popular ("old theory"), and what is the difference between them. Sometimes the difference is not much. Black has tried other systems against 6.Bg5, this might indicate dissatisfaction with the main lines, or it may be just an attempt to get out of book and confuse white.

Ok, thanks guys, appreciate it. Something to think about. I ran into this in Sam Shankland's repertoire book in the Sicilian. He usually goes in depth about differences in move orders, but in this particular case he just glances over it for some reason. Also the database showed a6-bd7 move order as the most frequent.

In the book of Wells and Osnos on Rauzer, the line 7. .... Be7 8. OOO OO  comprises 4 long chapters so many strong GMs have played it and its theory is developed. Each chapter is a different way for black or white to approach the position.  Thanks for asking the question! I haven't looked at these lines, esp the ones involving h6, for a long time and will check them again. The last game I played as white in these lines was in the 1970's vs Marty Appleberry, a very strong master. I tried a piece sac line which he refuted basically over the board. I hung on as long as I could and late in the game he missed a 2 move combination, that lost a pawn and later lost. Chess can be cruel. Blinking once can ruin an otherwise beautifully played game.

to prevent Bb5