That's not that easy. Thought I had a good one but forgot it again. So if someone else wants to take over, go ahead please.
While we're waiting for Shirley's answer here's another one: Hardly anyone in Germany (or perhaps Europe?) could tell you on the spot what this small loop you often find on the back of stylish men's shirt is for. Can you?
My curiosity made me go to Bing to find the answer to this. I couldn't wait any longer for somebody here to tell me. "Nothing these days. A long time ago people wore clothes a couple of days before laundering and men used a valet to hang their clothes on to wear the next day. The shirt was hung on a hook on the valet and there was a place for a jacket and pants, thus the shirt hook. The better shirts still add the hook but it’s for style rather than function."
My son has a couple with a loop and he has a habit of just hanging them on a hook as opposed to using a hanger.
You and @Shirley Martin got it right. Interesting, though, that American ladies, too, would be slightly at a loss for the answer. The loop was first used by US East Coast sailors who'd hang their shirts on ship loops. The loop became fashionable in the 1960 when university students used it to hang their button-down-shirts up in their lockers. That's why it's called locker loop. So your son uses it right - instinctively or because he'd learned it? My instinct had let me down I admit. There's also a romantic aspect. In American Ivy League culture, young women would rip off the shirt loops of the men they found attractive and tear the whole shirt I assume? Men would also cut off the loop of their shirts (thus saving the shirt) to show that they had a girlfriend.
Never heard of that one Thomas. Sadly, my son is sometime just to lazy to go to his closet to look for a hanger. But, interesting to learn about this.