On Duolingo, there is a question: "Why do the students need pencil holders, in fact?"

It gives as an answer: "Kial la lernantoj bezonas krajoningojn, fakte?"

Now, my first thought was that answer is wrong, because -ing- implies holding a single object and most pencil holders are designed to hold multiple pencils. Upon checking the meaning of -ing-, I saw "holder for one object (sheath, socket, stand)". I was feeling very satisfied until I looked at the examples and saw "plumingo, pen holder".

O-kay...feeling a lot less sure now. So I check -uj- to see if there is anything helpful there; and there's an example of "ĉapelujo, hatbox". A hat box. Something for carrying a single hat. Not ĉapelingo, but ĉapelujo.

Am I totally missing the logic of using -ing- and -uj-?

I would say a plumingo would not be a pen box (I'd say plumujo for that), but a kind of pen sheath. Like the ones at the bank, that hold a single pen attached to a chain. A pen holder versus a pen box, if you will.

-ing- usually does indicate that only one thing is held, but I'd think in terms of a sheath, the holder frequently matches the form of what it is holding.

If something was a specially shaped hat receptacle, roughly matching the form of the hat, I'd be more inclined to call it a cxapelingo than a cxapelujo.

I agree that the Duolingo course probably would have been better off with krajonujoj, since it is unlikely that a student will have a receptacle holding only a single pencil, as if in a sheath. Sounds very old fashioned, actually, like you have a thing with an ink well and a special cavity designed to hold your pen or pencil.

erinja:-ing- usually does indicate that only one thing is held, but I'd think in terms of a sheath, the holder frequently matches the form of what it is holding...

So, if you had a pencil holder, say, a chunk of wood, that has a single pencil-sized hole drilled into it. That would be a krajoningo, because the pencil fits the hole. However, if it had a single hole that was much larger than a single pencil, that wouldn't be a krajoningo, but a krajonujo.

What if the block of wood had multiple pencil-sized holes drilled into it? Could you still use krajoningo, even though there is more than one pencil-hole?

erinja:...I agree that the Duolingo course probably would have been better off with krajonujoj, since it is unlikely that a student will have a receptacle holding only a single pencil, as if in a sheath...

Oh, it does take krajonujojn as pencil holder as well.

00100100:

erinja:-ing- usually does indicate that only one thing is held, but I'd think in terms of a sheath, the holder frequently matches the form of what it is holding...

So, if you had a pencil holder, say, a chunk of wood, that has a single pencil-sized hole drilled into it. That would be a krajoningo, because the pencil fits the hole. However, if it had a single hole that was much larger than a single pencil, that wouldn't be a krajoningo, but a krajonujo.

What if the block of wood had multiple pencil-sized holes drilled into it? Could you still use krajoningo, even though there is more than one pencil-hole?

Yes, I'd call that a krajoningo.

erinja:...I agree that the Duolingo course probably would have been better off with krajonujoj, since it is unlikely that a student will have a receptacle holding only a single pencil, as if in a sheath...

Oh, it does take krajonujojn as pencil holder as well.

At least there's that. I understand why they did it though, there aren't a lot of common -ing- words and they were probably grasping for -ing- words using roots they have already taught. Kandelingo is a common one (and it does also include candle holders that have slots for more than one candle, akin to your pencil example above). Glavingo. Those are probably not words that they teach early in the course, if at all (kandelo and glavo).

The difference between -ing- and -uj- is much simpler: partial vs. full inclusion.
A krajoningo can hold several pens, as long as they stand out (protrude?). When you can fully cover them inside, it would be a krajonujo.

Quite right Kirilo.

That's why we say Anglujo rather than Anglingo. The English don't stick out over the English Channel.