I am facing a consistency issue when proofreading a paper regarding the use of the term 'time series'. When used as a standalone noun, it seems to be written as 'time series', with the two words being separate and having no hyphen. However, the individual has also used it as an adjective in the same paper, with the corresponding example being 'time-series problems'.

So is 'time series' hyphenated when used as an adjective? I could not find much reference or discussion about this on the internet, both seem to be in use, and I distinctly remember seeing the hyphenated version in many statistics textbooks and other papers as well, so I am now in doubt.

  • Admirable to strive for consistency. Do these related questions and their related questions help? english.stackexchange.com/a/34766/36710 english.stackexchange.com/q/11570/36710
    – livresque
    Jun 15, 2022 at 1:33
  • So I browsed through some of the links referenced in that thread, and came across the following: grammarbook.com/punctuation/hyphens.asp Rule 10a. Do not hyphenate proper nouns of more than one word when they are used as compound adjectives. So I suppose this might void the notion of a compound form vs. open form for 'time series'? Perhaps this question is entirely of stylistic nature, and it may not matter what is used as long as it is the same everywhere?
    – Triton84
    Jun 15, 2022 at 1:43
  • 1
    The rule is that modifiers of more than one word go after the noun they modify, but only one-word modifiers may precede the noun. The hyphen is to make one word officially one word. It's sometimes called "The Eleven-year-old Boy Rule". This is needed because intonation isn't represented in English writing; in speech this would not be a problem. So, to answer the presenting question, yes, time series is not hyphenated as a noun, but it is when used as an attributive adjective. Jun 15, 2022 at 14:36
  • 1
    I’m voting to close this question because "Do not hyphenate proper nouns" does not apply without proper nouns. Jun 28, 2022 at 12:50

4 Answers 4


The point of punctuation is to help readers more easily grasp the intended relationships between or among words or groups of words on a page. If a particular punctuation mark doesn't promote that goal, it doesn't serve any purpose beyond upholding some arbitrary rule of supposedly proper usage.

Oxford and Chicago on hyphenation of compound modifiers

Hyphenation in noun strings is a case in point: if adding a hyphen clarifies the relationship of the component nouns, the mark is useful; if not, it is superfluous. As The Oxford Guide to Style (2003) points out, the logical extension of the rule about hyphenation of phrases that involve either compound modifiers and a following noun or a single modifier and a two-noun phrase would entail clarifying the ambiguity with a hyphen in either case. And this is in fact what many writers used to do.

For example, if you were writing about a factory whose output was small you might refer to it as a "small-scale factory," and if you were writing about a factory that produced small weighing devices, you might refer to it as a "small scale-factory." In either case, the hyphen served "to unify the sense," as Oxford puts it, and thus "to avoid ambiguity.

In more recent times, however, the hyphenated compound subject has become less common, leading Oxford to provide this description of hyphen usage:

A small scale factory is a small factory that manufactures scales, while a small-scale factory is a factory that produces a small amount of something.

Applied to the posted example here, Oxford would presumably argue that time-series problems are problems that involve time series, whereas time series problems are series problems that involve time.

That's fine if "series problems that involve time" constitutes a significant category of things. But if it doesn't, in my opinion, requiring a hyphen for the case that is significant—"problems that involve time series"—amounts to needlessly punctuating the lily. A bit later in its coverage, Oxford essentially concedes as much when it observes that "scientific terms"—such as liquid crystal display and sodium chloride solution—"tend not to be hyphenated in technical contexts." But why not?

Two rationales for leaving compound modifiers open

There are, it seems to me, two possible explanations for this. The rationale from familiarity is that these phrases are immediately recognizable as set phrases and so do not require the adornment of punctuation to be completely and instantaneously comprehensible to readers; in such a case, the hyphen actually slows down the reader by needlessly pointing out the unitary notion of "sodium chloride" or "liquid crystal." The presumption tat a scientific readership would be quite familiar with set terms in their field seems reasonable enough—although the same could be said of businesspeople in their field, sports fans in their field, and artists in their milieu. The rationale from familiarity thus extends readily to any category of jargon or specialized vocabulary, and I see no reason why it shouldn't apply to nonspecialized terminology, too.

The rationale from what might be called nonconfusability is that there is no significant category of instances where "a crystal display containing liquid" or "a chloride solution containing sodium" might be intended instead of "a display containing liquid crystal" or "a solution containing sodium chloride." In the absence of any realistic need to provide for the possibility of, in effect, "liquid crystal-display" or "sodium chloride-solution," there is no practical need to hyphenate "liquid-crystal display" or "sodium-chloride solution."

Again adverting to the posted question, it seems fair to inquire whether (1) "time series problems" is a recognizable set phrase in the field in which the paper is written, and (2) whether there is any realistic possibility that "time series problem" might mean "time series-problems" rather than "time-series problems." If the answer to question (1) is yes, the hyphen is unnecessary as a matter of practical usefulness; if the answer to (2) is no, the hyphen is likewise unnecessary as a matter of practical usefulness.

Interestingly, The Chicago Manual of Style, sixteenth edition (2010), in a lengthy chart at 7.85, adopts a very rigid-sounding rule for handling phrases of the form "noun + noun, single function (first noun modifies second noun)":

Noun form open [e.g., tenure track]; adjective form hyphenated before a noun [e.g., tenure-track position].

And yet, under "chemical terms" in the same chart, we find this familiar example:

Open in both noun [e.g., sodium chloride] and adjective [e.g., sodium chloride solution] forms.

I can't help wondering how Chicago would have come out on "liquid crystal display."

AP on hyphenation of compound modifiers

For its part The Associated Press Stylebook (2007) seems quite untroubled by the lack of hyphenation in such terms that it expresses a preference for as air traffic controller, ball point pen, commodities futures contract, Saturday night special, snow avalanche bulletin, sport utility vehicle, stock market prices, television program titles, and winter storm watch. AP summarizes its general advice on hyphen use as follows:

Use of the hyphen is far from standardized. It is optional in most cases, a matter of taste, judgment and style sense. But the fewer hyphens the better; use them only when not using them causes confusion. (Small-businessman, but health care center.)

This is a far cry from the rule that a compound modifier consisting of two nouns should automatically be hyphenated when the modifier appears in front of the noun it is modifying.


In modern publishing, a great many noun phrases ("community newspaper columnist," "employee performance review," "football conference standings," "health insurance policy," "highway patrol officer," "hurricane probability assessment," "market research firm," "military deployment guidelines," "opinion poll respondent," "table tennis champion," "tax avoidance strategy," etc., etc.) tend to go unhyphenated, regardless of the quasi-rule laid down by Oxford and Chicago, simply because the publisher assumes that no meaningful ambiguity will arise from the lack of a hyphen. In this respect, the approach to the issue recommended by AP seems to be gaining ground in the real world against the (somewhat) contrary approach promoted by the university press style guides.

My advice is simply to try to be consistent with whatever decision you make about how to handle a particular phrase. If you start with "time-series problems," use it throughout; and if you start with "time series problems," try to stick with it as consistently as you can. (The problem here is that if additional modifiers appear in front of "time series problems," you may have to hyphenate "time-series" in those instances to avoid complications arising from the other modifiers—for example, "short time-series problems." But that's a whole nother can of worms.)

  • This has been covered here before, but I think the extracts above add some clarity, listing and in part explaining the modern trends. Jun 16, 2022 at 12:07

The most common use of noun phrases is to function as subjects, objects, etc. Therefore, when "time series" is used that way, it seems natural and no hyphen is needed.

Using a noun phrase as a modifier is less common, so we often use a hyphen to make it clear that it is a single phrase.

The hyphen can also resolve ambiguity. For example, "time series problem" could be interpreted as a problem that has to do with a time series or as a series problem that has to do with time. The hyphen makes the meaning clearer.

I think that it's fine to write "time series" when it has a typical nominal function (subject, object, etc.) and "time-series" when it has an adjectival function as long as you're consistent. Other people might prefer that it always be written the same way. There is no iron-clad rule.

(I see now that this question may be a duplicate. I'm willing to delete this if another question already contains a sufficient answer.)

  • Here's an example that agrees with you: timescale.com/blog/… But I was leaning towards no hyphen, so now I'm confused. Jun 15, 2022 at 5:24
  • @aparente001 People often do omit the hyphen, especially for common phrases when there is little likelihood of confusion, such as the title of the movie "The Glass Bottom Boat". Jun 15, 2022 at 17:25
  • You really should document your answer -- you're welcome to use the link I found. / Your comment was helpful for me and I suggest you expand your answer to include that. Jun 15, 2022 at 17:27
  • @aparente001 Thanks, I don't have time right now but will try to do so later today. Jun 15, 2022 at 17:34
  • It's not an emergency! It was just a tip for how to improve your answer. (By the way, a well-documented answer doesn't necessarily have to be long.) Jun 15, 2022 at 17:38

In a comment, John Lawler wrote:

The rule is that modifiers of more than one word go after the noun they modify, but only one-word modifiers may precede the noun. The hyphen is to make one word officially one word. It's sometimes called "The Eleven-year-old Boy Rule". This is needed because intonation isn't represented in English writing; in speech this would not be a problem. So, to answer the presenting question, yes, time series is not hyphenated as a noun, but it is when used as an attributive adjective.

  • If I were feeling provocative, I'd pick up on John's 'attributive adjective'. My own view is that the form/function debate is far from being happily resolved. Jun 16, 2022 at 11:58

The use of hyphens has considerably decreased over the last century or so, and many style guides now deprecate their use. When I wrote support publications for UK military equipment in the 1980s, we were 'officially' discouraged from using hyphens at all (which in many cases I considered detrimental to the reader's ease of understanding).

In this case, my own opinion is that the author has got it right, so as long as they have been consistent I would leave it unchanged.

  • "Can you clarify what sort of terrain we're going to encounter, sir? Was that 300-year-old trees, 3 hundred-year-old trees, or 300 year-old trees?" Jun 16, 2022 at 12:04

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