Q words are special,* and quoin is a particularly satisfying example, I think.
I'm sure you've seen them, even if you've never realised it.
Quoins are decorative blocks on the outside corners of a wall.
Can you see them?
Sometimes they give strength to a wall made mostly of stuck-together rubble, and sometimes they're just to show off.
The keystone of an arch can be called a quoin, too:
And so can a wooden wedge used to lock up type firmly into a frame:
Quoins are also wedges used to get a cannon to point up at the right angle.
Quoins: yes, they're all over the place.
And isn't it good to have a name for them?
Spot the Frippet: quoin. This word arrived in English in the 1500s. It's a variant of coin meaning corner.
*All right, you can spell this word coign or coigne, but it's not the same.