They appear to be best suited to fit 8.5 x 11 paper folded in thirds.
Why is that? I find folding in half to be much easier and would think that this would have become the standard. But it didn’t.
Maybe it’s actually quite easy for others to fold in thirds and I’m just the oddball?
Does it have to do with being able to seal letters with a wax seal?
Perhaps they had to adapt to early sorting machinery, which was sub-optimal (a la the QWERTY keyboard)?
Anybody have the straight dope?
I find folding into thirds easier. If you fold into halves, it doesn’t look right unless the corners match precisely (IMO). But when folding in thirds the outer part is generally shorter so the matching need not be precise.
I suspect the “third” size is a bit smaller and easier to handle, but it may well be historical. If you fold in thirds and leave the last fold smaller, then you can seal it with wax as you say and the whole of the letter is hidden. If you tried to fold in half, you’d either have to seal the edge or you’d have to fold in half so two edges met in the middle of one size. I suspect neither of those seals would be as srong.
An envelope made to fit 8-1/2"x11" paper folded in half would, if my thumbnail calculation is correct, use approximately one-third more paper than one made to fit the same paper folded in thirds.
My WAG(S)
An envelope with a document in it folded in half will be thicker on the folded edge, but one folded in thirds will be more symmetrical. This might help when sorting or stacking large numbers of similar envelopes. Also, a multi page document folded in thirds must come out of the envelope in one bunch. It’s possible to miss a page or two if the document is folded in half.
But really, its probably what KneadToKnow said.
When I was reading office administration I was taught that the middle fold should be slightly smaller than the other two. The reason being that large companies will use a machine to slice open incoming mail and leaving part of the page margins sticking out would ensure that the recipient did not inadvertently end up with two pieces of a letter.
A standard #10 business envelope is 4.125 inches high. That happens to be almost exactly the length of the palm of my hand. I can clench an envelope in my hand with my fingers securely gripping more than half the height of the envelope. That’s a much more secure grip than I have with a 6 x 9 envelope.
My WAG is carryability is the real reason for the envelope’s size.
In additon to other reasons listed, I suspect tsome of it has to do with a #10 envelope being a lot less flimsy than a 6x9" envelope. They are longer, but less flimsy when filled with a couple of sheets of paper. Those machines at the post office really kick envelopes through and they problably have to be programed for a lot less of a tolerance with that size.
I wouldn’t be surprised if this were the historical reason why documents were folded in thirds originally.
Just another WAG:
Security envelopes not withstanding, I could believe that if you fold the letter in thirds correctly, you could obscure the actual text and contents of the letter by the folding of the paper. That is to say, if you held the envelope against a light bulb, you would be less able to read the text than if it were folded in half.
Of course, this doesn’t count if people just open your mail. And again I must mention the caveat of security envelopes.
Also, in some countries, the half-fold envelope is considered oversized, and is charged additional postage.
I think the real question is, “Why is the 8-1/2"x11” sheet the standard for business correspondence?"
It probably has something to do with printing presses.
It isn’t, outside North America. In most of the world the standard is 210 x 297mm. Standard envelopes where this size is used also accommodate sheets folded in three, not two.
I suspect this could have has something to do with easy of handling the envelope. You can easily get envelopes which accommodate a standard sheet folded in half, but they are just big enough to be that bit more awkward to handle.
According to the the Wikipedia article on paper size it has to do with paper manufacture. 11" is “about a quarter of the average maximum stretch of an experienced vatman’s arms”. I have to say that looks like a fairly thin explanation to me.
I suspect the sizes today are what they are because of factors which mattered a bunch in the early 1800s, and which are all but incomprehensible to us today.
In other words, that 11" is 1/4 of the size of manually produced paper using 1810 technology is quite beleivable.
So look to history, not to current practicalities for your answer.
If you combine that Wiki article with this one on broadsheets it becomes a little more clear. What they taught me in my History of Journalism course was:
Start with a standard broadsheet, which was originally 34 x 22 or thereabouts.
Of course, newspapers have been trimmed down over the years.
Also, note that foolscap measured 17 x 13. Fold that in half and again you get 8.5". It appears that a 17" side of paper was really convenient.