It seems like every time I try to fold a piece of US letter sized paper in thirds to put it in an envelope, I totally bungle it. I’m incapable of eyeballing a third of the page, and using the size of the envelope as a guide doesn’t seem to help.

There must be some kind of clever trick to get even folds. I know an origami method to divide a rectangle into thirds, but that involves making little creases that would be aestetically unpleasing in a business letter.

Is there a foolproof method, or am I too much the fool?

Maybe if I get a job stuffing envelopes, I’ll get enough practice to master the eyeball method.

I usually don’t get it perfect… but always close enough. It’s all about proportions I guess. Take the bottom of the paper and bring it up until it’s about 1/3 from the top (or you’ve doubled up 2/3rds of the page at the bottom). I admit it takes some practice… but it’s not that difficult if you practice.

You could do one with a ruler to get it exactly perfect and then just use it as a model I suppose…

Fold one sheet perfectly, then keep it as a template.

Or, use a ruler: 11/3 = 3.667 inches = about 3-5/8 inches.

Oddly enough, my left index finger is exactly 3 inches long, and the base of the fingernail is about 1/2 inch from the end of my finger. I suppose I could use that as an eyeballing guide, if letter folding were a frequent task.

I don’t eyeball a third, which is indeed a bit difficult; I eyeball a halfway point, which you can do surprisingly accurately. As you make the first fold, the paper’s edge (specifically, it should be the bottom) should run down the center of the resulting shape. One side will be the top of the letter, the other half being the folded over bottom. Just make sure you are near to making the crease when you pass judgment, or the radius of the pre-fold with throw you off.

I just did it, quickly and without special care, and was off by less than 1/16”.

Measure the aforementioned 3 5/8" from the tip of your middle finger, and tattoo a line across the back of it at that distance. Permanent portable yardstick for that all-important first fold.

If you’re not interested in tattoos, measure various body parts to try and find that 3 5/8" measurement (length of fingers, width of hand or combinations of fingers (i.e., my left middle finger is that long)), then use that as your measurement.

When I had to do this regularly, I cut two small and barely noticeable notches in the front edge of my desk. One notch was to locate the top edge of the paper, and the other was the location of the first fold. It made things much easier.

Put the envelope next to the piece of paper. Fold the paper from the bottom so that the distance from the fold to the top (formerly the bottom) edge of the folded over part is about 1/4" less than the height of the envelope. Then fold the remaining top portion over the part that is already folded.

This may not fold the paper in thirds, but it guarantees that the paper fits in the envelope nicely.

I do as Waverly does, more or less. It helps if the paper is lined, of course, but even a piece of blank printer paper isn’t that hard to get pretty close to even thirds if you do it this way. Because the crease can throw me off somewhat, I tend to fold the top down first, and then fold the bottom under it. Not quite as precise as Waverly, but easily within 1/8" on unlined paper.

It is impossible. If I’m not mistaken, this is a form of the old “trisect the angle” problem, which has been proven to be impossible with just a compass and straightedge. Myself, when I’m folding origami that calls for trisecting an edge, I just bring the edge into an S-shape and slowly squeeze it while wiggling it back and forth to line it up better. To do any better than that, you’d have to use a ruler.

I have a plastic doohicky I found at an antique store about 10 years ago. It is about 4 inches by 10 inches and was made by the Globe Company of Chicago, Illinois. It has marks that line up with the corners of 3 different sizes of paper. You lay on top of the piece of paper, line it up with the corners of the paper and lift the paper and crease along the edge of the doohicky. Everyone that receives a letter from me has to be impressed with the precision folding job.

Actually, it’s a completely different problem, for a couple of reasons:

Here we’re using origami, not a compass and straightedge, so there very well may be a method of trisecting an arbitrary angle with origami (I’m not actually sure offhand if there is or not, however).

We’re trisecting a line segment, not an angle. This is certainly possible in both origami constructions and compass and straightedge constructions.

I know of an origami method for folding a square into thirds, and I figure not much would be required to extend it to a general rectangle:

Orient the square so that one edge is toward you. Take the bottom right corner, and bring it up so that it meets the middle of the top edge of the square. Flatten and fold. The intersection of the (former) bottom edge and left edge of the square is now exactly one third up the left edge. (This is known as Haga’s theorem).

If you just almost fold it without making creases in the paper, you can get it lined up before you commit to making creases. I believe erislover and caracal are describing the same process.

A quicker way (which I was taught in my high school typing class, and in which we used honest-to-gosh typewriters, which will give you a clue as to how long ago it was) is to put the envelope down on the table with the flap facing up. Then slide the paper under the flap, getting it snug against the top fold of the envelope. Now bring the bottom edge of the paper up until it’s just a bit above the bottom of the envelope and fold. Now make the obvious second fold (i.e. at the place where the bottom edge of the paper now is), and you’re done.

Well, don’t know how impossible it is, but I did some quickie math and then took out a piece of paper to check out the method below. It works. It does, however, require you to estimate the middle of a length. Let’s say you place the sheet of paper in front of you with the long side running left to right. Fold the left side over to the right and line up the edges to locate the midpoint of the length – don’t make a crease, just leave your index finger along the bottom edge where the midpoint is. Now fold the left side over so that your index finger splits the length of the folded-over section. Make the crease. How good this turns out depends on how accurate you are at eyeballing the midpoints. I found it pretty easy and quick to do. The math proves that this will fold the paper into exact thirds.

Wow! Cool…
I just compared my index finger with a machine-folded letter and found that the distance from the crotch of the finger to the tip is precisely the right length. I just folded three perfect letters by resting my tentative crease in the crotch of the finger while making adjustments.

Crap, I was sure I posted this before, but I tried Waverly’s method (imagine a line halfway, then fold up the bottom flap so that half of it will be above the imaginary line and half will be below) and it worked beautifully. It’s much easier to eyeball two halves than one third!