I have never in my life seen this either. I think it might be a US only thing?

A duvet is a thick bed covering, kind of like a thicker version of what is called a comforter in the US. They can be really comfortable, especially if you like your covers to weigh a little more, but it’s a hassle to wash and dry them. To reduce how often you have to wash the duvet, removable covers fit around the duvet. Conceptually it’s kind of the same reason we put pillow cases on pillows.

Because of how large and floppy the duvet is, it’s kind of hard to get them all the way into the corners of the covers, and if you don’t have them in the corners well they can migrate around and bunch up on one side. To help combat that some duvets and covers have little straps in the corners you can tie together to keep everything in place.

And if they don't have this. Hand sew a shoelace to the inside of the cover, and just tie it to the corner of the duvet. Shoelace knot about an inch down the corner tight, and it won't go anywhere.

Both my duvet and cover have little loops. But one has loops that are just long enough to pull them through the other and tie a knot. Works well enough. I feel like some country that uses a lot of duvets should set a standard requirement for all duvets to have loops and all covers to have ties.

Oh nice. I have these ancient Ikea covers that are super old but my dog likes to dig in my bed so I hang onto them because it's ok if they get ripped.