Question: Oftentimes a bishop will move to b5 or g5 early in the game.

[![enter image description here](https://i.stack.imgur.com/4R8sa.png)](https://i.stack.imgur.com/4R8sa.png)

I personally never do this because I see a6 coming, and if I move the bishop back to a4, then b5, nearly trapping the bishop, and certainly having made it a waste of time to move it to b5 in the first place.

However, when I watch games by top-level players, bishops get moved to the 5 row all the time, and nobody ever fights them away with pawns like I think they will. Why not?

Answer: 
Here are a couple of relevant questions and answers that might help to explain why the bishop move happens:

* [Morphy Defence in Ruy Lopez](https://chess.stackexchange.com/questions/4733/morphy-defence-in-ruy-lopez/4734#4734)
* [Why is the Ruy Lopez such a common opening?](https://chess.stackexchange.com/questions/226/why-is-the-ruy-lopez-such-a-common-opening)

In summary the main points seem to be that:

* Black loses as much time chasing the bishop as White does retreating it
* White can gain a long-term advantage from Black's weaker Queen side pawn structure

As you might expect in such a popular opening though it's very finely balanced for both sides.

If as you say you are *not* seeing a6 and b5 in the games you're looking at, you might be looking at games where Black plays the Berlin Defence: 3... Nf6. This is quite popular at the moment in very high level games. 

If you want to see some games where the pawn move a6 is played, "putting the question to the bishop", here's a link: <http://www.chessgames.com/perl/chessopening?eco=C70>
One downside to expanding the pawns too much before developing pieces is that the pawns can become overextended and weak. Below is a sample variation of how Black can become overextended:

```
[Fen ""]

1. e4 e5 2. Bb5 a6 3. Ba4 b5 4. Bb3 c5 {Intending to trap the Bishop in a 'Noah's Ark' with c4.} 5. Bd5 Ra7 6. a4 {Trying to break the queenside pawns up and make the vulnerable.} b4 7. c3 bxc3 8. Nxc3

```

In this sample variation, White leads in development and Black's space advantage disappears.

---

N.B. The sample variation was worked out in my head, and references no games in particular, nor do I examine the best moves. 4... c5 is a mistake to show the perils of overextending, and 5...Ra7 isn't forced.