Helge Kreutzmann writes: > Sigh, learning more and more the finer details of English. > > I just wonder, since these are technical documents, if this is > supporting understanding or reducing it? Personllay I prefer one word > for one concept. There's a commonly-suggested rule in style guides that "open" compound nouns (those still written as multiple distinct words) should be hyphenated when used to modify another noun, but not when an open compound noun occurs as a noun by itself. So for example, we might have "Richard Stallman wrote several free-software licenses", but "Richard Stallman originated the modern concept of free software". This is described, for example, at https://www.merriam-webster.com/words-at-play/hyphen-rules-open-closed-compound-words although this source is less prescriptive and definitive than some others. That source also describes a trend where newly-coined English compounds are usually regarded as separate words for some time, but people increasingly see them as single words (with a multiword etymology) after acquiring greater familiarity with them. In this case, it seems that the man page editors feel that "run level" is too unfamiliar or recent as a technical term to be widely understood as a single word (so my intuition -- that it could be a single word -- is, in a way, further along in the process). The Merriam-Webster article seems to imply that, after a few more decades, more people may become comfortable with "runlevel" as a single word, assuming we continue to use that same term frequently for the same concept. The hyphenation rule for an open compound (in the sense described in the Merriam-Webster article) modifying another noun, as opposed to an open compound standing alone as a noun, is not completely universal and might not be understood by all English learners, but it's a rule that's very, very commonly suggested by teachers and grammar references. It would not be uncommon for both native and non-native English speakers to have been taught some version of this rule in school at some point. We might sort of analogize it to the German rule for the case where a proper name is used as a part of the proper name of something else (like a street, school, or prize). There German expects to hyphenate the entire resulting phrase, like Heinrich-Böll-Preis Kaiser-Wilhelm-Universität Karl-Marx-Allee Käthe-Kollwitz-Preis Max-Ernst-Stipendium Richard-Strauss-Straße Sankt-Johann-der-Täufer-Kirche even though the original proper names (Heinrich Böll, Kaiser Wilhelm, Karl Marx, etc.) aren't hyphenated when used in their original sense to refer to _people_. While this English rule isn't exactly the same, it also shows a propensity for using a hyphen when a noun is used in one context, but not when the same noun is used in a different context.
