Something I’ve noticed on a number of my button-down shirts - the bottom button has a buttonhole that is horizontal, not vertical like the rest. What’s the reason? I’m guessing it has to do with that part of the shirt usually being tucked in. Does that somehow make that button less likely to come unbuttoned? I usually don’t bother buttoning it anyway, so not sure what difference it makes.
I just noticed that myself a coupe of weeks ago. Only on 1 shirt. It also has dark thread - the others light.
My only guess is it is a guide to ensure you don’t misbutton - assuming you button from bottom up and pay attention. Never noticed it before. Weird.
Maybe youngsters these days are stupider than us oldsters, such that they need specific instructions for how to dress themselves! 
If you are talking about the hole at the very bottom that doesn’t have a button on the opposite side, some dress pants have a little interior button the you hook through that hole to secure the shirt and keep it tucked in front.
“In most shirts, there is more likely to be horizontal stress/pulling than vertical stress/pulling. Buttons are more likely to stay in the buttonhole if the direction of the stress is in the same direction of the stress (button pulls to end of buttonhole) than if it is perpendicular (button pulls out of buttonhole). So horizontal button holes are actually more functional than vertical.”
From a Quora posting here
If that’s the case then why aren’t all the buttonholes horizontal?
A thread we did a few years back in which misreading of the subject line ensued.
Again, if we are talking about that lone hole at the very bottom with no button on the opposite side, then a possible explanation is that it is easier to attach it to the interior pants button if horizontal instead of vertical.
That would be madness!!
BTW, I have several good dress pants that have that little interior button to attach the shirt to.
I used to as well. Then I got rid of “dress pants” and used “dress shirts” for lawn mowing. I’m happier.
Czarcasm this is not a buttonhole without a button. It’s the bottom button + buttonhole, but the buttonhole is horizontal rather than vertical like all the ones above it.
Not sure I’ve ever seen something like what you’re talking about - a buttonhole on a shirt that buttons to a button inside the pants? I’ve seen a button on the inside of dress slacks near the top of the zipper, but there is usually a cloth tab with a buttonhole on the other side of the zipper that it buttons to.
I have several dress shirts built exactly this way, particularly ones from Land’s End.
I’ve hypotheisized that the bottom button, with a traditional vertical buttonhole, is more likely to become unbuttoned, particularly for men who have a bigger belly, hence the choice to make that particular buttonhole horizontal.
I always thought it was an attempt to emulate dress shirts from another time that did attach to your pants. But now they are mostly to show that the shirt is of higher quality. Along with the expensive Polo or Nautica insignia crap. It serves as a bottom button, but I usually start buttoning my shirt from the top, feeling for the one at the neck and then going down one.
"However, vertical button holes look better in general. Especially so if the button is being pulled to the end of the buttonhole.
So convention has been to use vertical button holes except where the button hole is likely to be out of sight. This is typically the bottom most button hole (tucked into your pants) and sometimes the topmost button hole also (under your tie, or unbuttoned)"
From the same Quora link as above.
For some reason the idea of attaching your shirt to your pants seems bad to me.
Maybe it’s one of the reasons men wore a lot a dark suits.
Aesthetics? For most people, the torso is more vertical than horizontal and the buttons as a whole form a vertical line. The bottom button is usually not visible on a dress shirt.
Next you’ll be saying cats and dogs should be living together!!!
I’m pretty sure it’s to keep the button buttoned. The last button does tend to come unbuttoned when tucked in. Works for me anyway.
A friend who deals in vintage Hawaiian shirts says that the mark of a true vintage shirt is that the buttonholes are all horizontal.
Very bad idea, especially when answering an urgent call of nature.
I’m used to women’s clothes that have odd closures. Shirts that have a snap on a thread that you hook around your bra strap to keep the straps in place, side zips that can be hard to reach, wrap dresses that can unwrap, oddly placed slits that will expose upon sitting.