Feel the seam in the jeans your wearing. Start about at the hip. Now run your finger down the seam. Feels good doesn’t it? Go ahead, I’ll wait.
Feel how the seam overlaps on one side. Okay, now keep running your finger down.
Uh oh.
For every single pair of pants I’ve ever worn there is a section of the seam, usually about mid thigh, where the seam flips and the part that overlapped is now on the bottom and the other side is overlapping it.
Why does the seam flip?
Well how about that, you’re right. My jean’s seam flips mid to high calve.
I’ve noticed this too. But the black jeans I’m wearing right now most definitely don’t have the flip.
I’ve always assumed it was because I mostly buy jeans on sale, and the ones with the flip are seconds, and can’t be sold full-price, so they go into the sale bin.
I’m not sure what you mean by “overlap”. I’m wearing dress pants right now, so maybe that’s why I’m not seeing/feeling it. Is it only on jeans? What about khaki/Docker’s-style pants?
Perhaps I’ll walk around the office and ask my coworkers if they can feel it on my pants. “No, no, try the one on the inside of the leg…” I’ll post the results from HR.
The seams on jeans are called “french” seams. I don’t know why. I’ve never noticed any "flipping’ on the 501 Levis I’ve ironed for years.
Cheaper jeans have french outside seams and flat inside seams.
The seam in jeans is a flat felled seam, in which the to ends of the fabric overlap instead of butting together at the seam as they do in dress pants. I believe khakis and Dockers have the same construction.
Huh. I’ll have to check my jeans when I get home. This is really interesting.
I have often noticed this. Every pair of jeans has it*, even expensive £150 designer-name ones. I think I’ve actually meant to post a GQ thread to ask about it, too!
I have no idea, but there must be a reason for it - it’s definitely not a “fault”.
I have a pair of jeans in my drawer here. On the right leg, the seam on the outside goes “up” from left to right all the way down the leg until about five inches from the hem, at which point it flips over and the seam now steps “down” instead of up.
The left leg is a mirror image - the seam steps “up” going from right to left (i.e. from the front panel of the leg to the back panel) but this time the flip-over point is even closer to the bottom of the leg - about 3 or 4 inches.
Most jeans I’ve seen have the flip-over point higher up, closer to mid-leg.
Can a clothes designer please tell us WHY??? 
That overlap french seam you are talking about is the inseam. The seam in question is the outer leg seam and is not constructed in that fashion.
It is done on a serger like machine. This machine puts in a line of stitching, (creating the seam), as well as an overlapping stitch (that keeps the fabric from fraying), as well as trimming off any excess cloth, and leaving a tidy seam, (which you would see, should you look inside your jeans), all in one pass.
Most dress pants are not constructed in this way. Instead each piece of fabric is separately overstitched to stop fraying, before being sewn together. During construction this seam is actually pressed open, (which you would see if you looked inside your dress pants).
In dress pants, this construction allows some play at the seam and allows the fabric to drape and hang, straight and true. The construction of jeans, with the seams overstiched together interferes with this slight bit of give. Especially where the curve over the hip changes to the straight of the leg. That little bit of missing ‘give’ manifests itself in a flip of the lie of the finished seam, going to one side at the top and then switching where the seams goes straight.
Don’t know if that makes sense or not but there you have it.
Huh. Weird. And on the jeans I’m wearing now, the flip occurs in different places on either leg. Right leg the flip is about mid-calf. Left leg the flip is mid-thigh.
Now this is going to be bugging me all day. Thanks.
The flat-felled seam is that strong, double folded seam with two parallel rows of stitching. It’s usually on the inside of the leg. The other side is a simple seam, because doing a flat-felled on a closed leg is nearly impossible.
The heavy material used in jeans is just ornery to work with. The outside seam is made with the pants inside out, then the loose flaps are sewn together with an anti-ravel stitch. The resulting flap lies whichever way it wants. The seam isn’t made like the one in your dress pants, because denim just won’t do that, even if you beat the fold with a mallet.
It would be picturesque to say that denim is unruly because it hangs out with miners, cowboys, and movie tough guys. It wouldn’t be true, though. It pulls, shrinks, and wiggles on a bias. Get the whole seam to lay one way? Hah. It’s like telling Shakira to stand still.
On preview, I see that elbows said pretty much the same thing, and probably more precisely than I said it.
Mine’s mid-thigh. Reminds me of how they fixed Beck Weathers’ nose.