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: 
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.
"Spanish Bishop" can become very useful and strong piece, especially when it arrives on the c2 square sometimes.