Japanese protocol for business letter?

Question:

Hi I'd like to know what the rules are for folding business letters in Japan.
I've heard they're very strict with how the letters are folded and inserted into the envelope.
When folding the paper does the letterhead (top of the page) fold inside , or is it supposed to be on the outside or top? Please if you have any sites you can recommend?

Answer:

The most common way is to fold it into 3 tiers. But this is the case only if the letter and the envelop are at certain size. It depends on each case after all.

Addition:
>A4 size paper and 120x235mm envelope
I think that's the most common size. So use the 3 tier method.

>and is the letterhead supposed to be on the outside or the inside (for 3 tiers)?

It does not matter so much. But don't put the printed side (side with articles) at the outside of folding, right?

This page shows simple image of 3 tier folding.
http://letters.tank.jp/manner/