Opening Principles... why?

I've heard opening principles and have seen how they work, but I'm not sure where they come from. I understand some things like central control, but I'm still confused about these issues:

Why knights before bishops? It seems to me that as long as your pieces get developed, the order shouldn't matter.

I've heard move knights to f3, c3, or d2. Is there something inherently wrong with a knight on e2?

I know you don't want to move too many pawns, but is it worth moving pawns to a3 or h3? It can prevent a knight or bishop on b4 or g4, and gives a bishop on c4 or f4 somewhere to run.

Any help is appreciated.

Knights before bishops:

As you point out, there are some well known squares where your knights are going to go. It's more difficult to know where your bishops are going to go. Are they going to go pin a knight on b5 or g5? Maybe you will fianchetto them on g2 or b2, or perhaps they will defend a center pawn, or point out through your pawn chain from e3 or d3. Maybe you want a bishop on c4 to attack f7. So sometimes it's better to wait on the bishops so you can develop them better.

There's nothing inherently wrong with a knight on e2. But if you're going to castle kingside, the knight on f3 protects the h2 square. Notice also that if you have a knight on e2 and it gets pinned by a bishop on g4 playing f3 to kick away the bishop weakens your king.

In some openings a3 and h3 to keep your opponent's minor pieces out of b4 and g4 are good moves. But in most openings it's not necessary and you can move a3 or h3 after your opponent puts a piece on b4 or g4. If your opponent moves his piece away from that square then he's used 2 moves to your 1 so you gained a tempo. If you use a3/h3 as a preventative measure then your opponent has gained a tempo. It's very difficult to determine from scratch whether this is necessary -- sometimes you don't want to allow the bishop to pin the knight because it will take it if provoked. Since the majority of cases you don't want to push these pawns, you get a general guideline not to.

the main idea of an opening is getting prepared for the middle game.
Obviously development is the main issue.
The best way to develop is bringing your minor pieces to their optimal positions.
and your minor pieces should support the center and not be in it.
Also, it appears the best way of controlling the center is having a majority of pawns, but no more than 2.
from white 2 pawns on the 4th line is optimal, and for black 2 on the 5th.
maintaining your pawns there is important as well. these will be support for your knights and positions. One rank further and their utility will be lessened by the stronger opposition of the 2nd rank.
knights are only useful on good posts because of their limited range, also, their special attacking abilities makes them more valuable on advanced lines.
bishops should be attacking or supporting the center from a safe position if possible, as in behind your own pawns.
the hardest is getting knights on good positions, bishops can be useful from the 2nd rank.
after everything is developed well and towards the center, you can plan an attack according to where your pieces can create a majority.

besides this, it is important to develop as smooth as possible, keeping tempo is important. If your own pieces stand in the way, your opponent will take the advantage, and have the center. 

-knights before bishops is more logical as i said before, bishops can function from within your own side as well.

-the position of the knights should be thought of by supporting the center

-preventing moves which are not threatening, costs tempo. I would not suggest h3 or a4 unless they are prior to b4 or g4. They can also be good if your opponent has too much pressure on the given point.

I hope this was useful,

The knights really belong on the f3/c3 squares. The main reason the queen's knight goes to d2 is to not block the c pawn in d pawn openings. Fluidity of the c pawn is important to maintain or attack the center, in the case of c3 or c4, respectively.

It's not very helpful, but you should only play a3/h3 when black's bringing the bishop to that square would be putting _too_ much pressure on your position. Here is an example:

1.e4 c6 2.d4 d5 3.Nc3 g6 4.Nf3 Bg7 5.h3

This is a good move because black's plan revolves around the central tension, and if he is allowed to play ...Bg4 then this will further that goal.

But, in this case, it is probably not a good move.

1.e4 e5 2.Nf3 Nc6 3.Nc3 Nf6 4.d3 d6 5.h3?!

I don't think it is a real threat in that case, mostly because white's light squared bishop is inside the pawn chain, and white could answer ...Bg4 with Be2.

It's also important to understand that the opening principles aren't absolute. You don't always have to develop you knight before you bishop, and sometimes it's okay to move a piece more than once.

Knights are developed first because they are much slower pieces than Bishops and take longer to reach where they're going.  A Bishop can travel long range and be developed instantly. Knights placed on c3, f3,c6, f6 squares have good scope they cover the center of the board both chess players dispute control over.

 A very good answer to all the questions, a couple things I would add:

- the knight on f3 - assuming you castle kingside - not only protects h2 but also prevents an attack on g2 along a diagonal.  A kingside with a knight on f3 and all 3 pawns on their original squares is very stable and difficult to attack.

- anytime one of these kingside pawns nudges forward, with the exception of making room for a fianchettoed bishop, it weakens the kingside.  The move h3 inherently weakens the g3 square ... permanently.  Sometimes it's better to tolerate a temporary pin than kick the bishop out and certainly using the move preventatively has the same problem.

But as others have pointed out: don't be mechanical.  There are always exceptions.

 Thank you for the examples that went along with the advice on advancing the h pawn.  The visual aid was very helpful. 

Thanks for the feedback everyone, it's a big help.

Keep it going guys. Very informative and very important. Thank you.

Although the reasons given above are very logical, I don't think a rule like "Knights before Bishops" is really useful. I can't remember a situation where these were the two options, and I chose between them using this rule. It depends too much on the particular position. You have to have a concrete plan, and then you pick the move that helps best to implement the plan, or works against the opponent's plan.

Same for moves like h3, a3 et cetera. All moves cost a tempo, if you move h2-h3 you can't move another piece. So it depends on whether that move does the most at that time, to implement your plan or obstruct the opponent's plans. Sometimes it does, sometimes it doesn't. With pawn moves this is just slightly more important, since all pawn moves have a drawback - you can't move them backwards, so they leave behind a weakness. And that counts double if the pawn is in front of your king. But general rules in chess are hardly ever actually useful.

You're looking at it the wrong way.  The rule doesn't cause the Knight move to be better, ever.  The fact that the Knight move is more often correct causes the rule.

Sure, but that by itself doesn't mean it's a useful rule. A rule is only useful if you can use it to decide which move is best, over the board. And this is one rule I've known about forever, but never actually used.

Edit: although it's of course possible that I'm using it subconsciously, or using a variation like "well, at least it's clear that my knight is going to go to f3 here but I like to keep options open for the rest, let's play Nf3". But that's different.

If you are going to make bold statements that go against the logic provided by masters, please provide specific examples that reinforce your point.  It is clear to me that knights are most often developed first to defend and attack the center.  The bishops are not best suited for this role positioned on e3 and d3.  If fianchettoed, I can see why the bishop would be developed first.  Can you think of any examples from your games and research where developing a bishop first or disregarding the general rules was advantageous? 

These "rules" are to be used more as a guideline and not as strict dogma. The rule about knights before bishops is simply because the bishops usually have a wider choice than the knights. The optimal squares for the knights ( in the opening) are easier to determine than for the bishops. 

You're still looking at it the wrong way.  Do you more often devellop your Knights or Bishops (whatever your reasons for doing so)?  If Knights, you are following the rule, even if you (hypothetically) had never heard of it.

It doesn't go against the logic of masters at all. I've never seen a game annotated by a master that said "I played Nf3 here because of the knights before bishops rule".

There are plenty of perfectly normal openings where one side develops a bishop first, and I have a feeling that openings where both sides play both knights before the first bishop are quite rare! So rare that I think they're all named "Four Knights Variation" and the like.

1.e4 c6 2.d4 d5 3.Nc3 dxe4 4.Nxe4 Bf5.

1.e4 c6 2.d4 d5 3.e5 Bf5.

1.e4 e6 2.d4 d5 3.Nc3 Bb4.

1.e4 e6 2.d4 d5 3.Nc3 dxe4 4.Nxe4 Bd7.

1.e4 e5 2.Bc4.

1.e4 g6 2.d4 Bg7

1.e4 e5 2.f4 Bc5.

1.e4 e5 2.f4 exf4 3.Bc4.

1.d4 Nf6 2.Bg5.

1.d4 d5 2.Bf4.

1.d4 d5 2.c4 dxc4 3.e3 Nf6 4.Bxc4.

1.d4 d5 2.c4 e6 3.g3.

1.d4 e6 2.e4 b6.

1.d4 f5 2.g3.

Of course, there are more common variations where both sides do developed a knight first. Still, hardly a rule that's useful in practice, to choose a move. It's more of an observation.

I'd recommend learning an opening repertoire that goes 4-5 moves deep. That's not that hard, and you'll almost always at least have your first piece out by then. And from then on, the "rule" is obsolete.

Answer: it depends on the System you're using.

Classical Systems dictate Knights on c3 and f3, and avoid a3 and h3's tempo loss.

Hypermodern Systems dictate otherwise. 

Which is better? Either.