Shouldn't it be kyoukaisen-jou_no_horizon? 境界線 means "boundary line" and 上 is a suffix meaning "upon". Splitting it as 境界 (kyoukai) + 線上 (senjou) is incorrect. Or do we just not care because the majority of the English speaking internet romanizes it as "kyoukai senjou"? If so, at least an alias would be nice I guess.

Since I can't really add much into this kind of issue, not knowing the language, I did at least pull up that on wikipedia they do use "Kyōkaisen-jō no Horaizon" for the translation on the English page and ANN when it first announced the series getting an anime used "Kyōkaisen-jō no Horizon" for the title. Hope that offers something.

I'd suggest kyoukaisenjou_no_horizon myself. Partly on subjective aesthetic grounds and partly because I think an argument could be made for consistency with g-senjou_no_maou.

Language-wise, it's just one of those occasions where the concepts of suffixes and word boundaries as we understand them in English break down. 上 is something you can add at the end of any other word to express the notion of uponness/aboveness in relation to that word, but the same is true of 線 to express the notion of a line. And 線上 is in dictionaries as a word itself, which is probably where the confusion comes from.

Frankly, it strikes me as equivalent to debating whether the better English translation would be 'on the borderline' or 'on the border line'. Which is why I make my suggestion on aesthetic grounds.

That's not a very equivalent comparison. Both "on the borderline" and "on the border line" would be converted into the same syntax tree: "(on (the (border line)))" vs. "(on (the borderline))". In the case of 境界線上, writing it as "kyoukaisen-jou" analyzes it as "(above (line-of (border))" where as "kyoukai senjou" analyzes it as "((above line)-of border)" which is different, and IMO incorrect.

Consider the compound word with particles inserted: 境界線の上 vs. 境界の線上. I think you'll agree that the former is a better representation of what 境界線上 means, and also makes more sense to say, both in the context of the story and generally.

Writing it as "kyoukaisenjou" would just preserve the Japanese spacing, i.e. no spacing. That's fine with me too.

I didn't mean for it to be a perfect analogy, just to illustrate approximately how consequential the matter is within the Japanese language itself. 境界線上 describes the same fairly simple concept no matter how you slice it, and I doubt that anyone content to let it remain in kanji has thought about whether it contains one, two or three distinct units of meaning and what those are.

That's the whole point - in Japanese there are no spaces, but when romanized there are. I suppose a simple policy would be to just write all such compound nouns as single words.

Spacing and hyphenation is always a bit of a headache. So ignoring it by compounding the words works...

Let's go with kyoukaisenjou_no_horizon and I'll alias these to it:

create alias kyoukai_senjou_no_horizon -> kyoukaisenjou_no_horizon
create alias kyoukaisen-jou_no_horizon -> kyoukaisenjou_no_horizon
create alias kyoukai_sen-jou_no_horizon -> kyoukaisenjou_no_horizon
create alias horizon_in_the_middle_of_nowhere -> kyoukaisenjou_no_horizon
create alias horizon_on_the_borderline -> kyoukaisenjou_no_horizon
create alias horizon_on_the_middle_of_nowhere -> kyoukaisenjou_no_horizon
create alias horizon_over_the_middle_of_nowhere -> kyoukaisenjou_no_horizon
create alias /ksnh -> kyoukaisenjou_no_horizon

create alias /ksnh -> kyoukaisenjou_no_horizon

But using /ksnh instead of /kjnh implicitly favors "kyoukai senjou" over "kyoukaisen-jou"!

Just kidding :) Thanks.