I realised this morning that I appear to have made a habit of the Ian Knot—not only do I start tying it automatically when I want to tie my shoelaces, but I don't even have to think of the finger movements specifically any more.
I remember a while back reading about alphabets used for various languages spoken in the former Soviet Union; they often contained letters which the table said were used only for writing loanwords from Russian.
I thought that was rather quaint... but recently, I've thought about the situation in German, and I think it has a couple of them, too. (And while typing this up, I realised that Latin was in a similar situation, with three letters—C K Q—for one sound, though Latin ended up splitting them up for different uses.)
Specifically, I couldn't think of any "native" German words with the letters X and Y in them; English, on the other hand, used those letters for native words as well (wax, by) while German goesn't (Wachs, bei).
Also, C is arguably a "loan-word-only" letter in German; in native words, it occurs only in the combinations ck ch sch.
Which probably explains why C X Y are used in German Grade 2 Braille for letter combinations (en mm el), since the letters themselves aren't common (and if you want to represent them, you have to "escape" them with a dot-6). (Q is also used, for "ll", since while Q occurs in native German words, it's not that common.)