Introduction and origins
What’s the difference between continual and continuous? There’s a short answer, but it’s misleading, so – surprise! – I’m going with the long and complicated one.
Some people make a firm distinction between the two adjectives, but others don’t or only sometimes do. The distinction has merit, but it’s not categorical, more the codification of a general but lopsided pattern.
Because the words are so close in sense and use, they’re often used interchangeably (the adverbs continually and continuously even more so). This seldom leads to confusion or difficulty, but it’s also true that each word has domains it specializes in and others it’s less suited to.
Both words come from Latin continuus ‘hanging together, uninterrupted’, continual arriving via Old French continuel. Their endings, –al and –ous, are common adjective-forming suffixes. The words’ more recent history sheds light on their use, but first let’s look at how they’re defined, since this reflects how they’re used and gets to the centre of the problem.
Definitions and history
Six major dictionaries – M-W, AHD, and Dictionary.com in US; Collins, Macmillan, and Oxford Learners in UK – each offer two definitions of continual. These vary in style and emphasis but can be condensed to: 1. recurring often; 2. continuing without interruption. Significantly, in three cases the ‘recurring’ sense comes first, in three it comes second.
A couple of dictionaries note that what is continual is often an annoyance. Later we’ll see why. Where synonyms are listed, continuous is often among them.
The same dictionaries define continuous (M-W, AHD, Dictionary.com; Collins, Macmillan, Oxford Learners) less uniformly, with 2–4 senses, some in maths or grammar. The non-technical senses centre on something continuing or being uninterrupted in space, time, extent, sequence, etc., or being connected in an unbroken line or surface.
So there’s a lot of overlap, but it’s asymmetric: continual strays into the semantic space of continuous, but the sense of ‘recurring’ is generally limited to continual. (Even so, Oxford Learners includes ‘repeated many times’ as sense #3 of 4 for continuous.)
Continuous is more physical and spatial than continual is, befitting its origins in science. Take this line in Darwin’s On the Origin of Species: ‘In most cases the area inhabited by a species is continuous.’ Robert Burchfield, for decades an editor at the OED, summarizes the two words’ historical development in his 1998 revision of Fowler’s:
Since it was first recorded in the 14C., continual has been used at all times to mean ‘incessant, perpetual’, and also, less strictly (OED), to mean ‘repeated with brief intermissions’. Continuous is a more modern word (first recorded in 1673) than continual. It was first used in technical contexts in botany (of plants having their parts in immediate connection) and optics, and was not brought into general use until the 18C.
Burchfield endorses Fowler’s original Dictionary of Modern English Usage (1926), which describes the distinction awkwardly but still fairly accurately:
That is –al which either is always going on or recurs at short intervals & never comes (or is regarded as never coming) to an end. That is –ous in which no break occurs between the beginning & the (not necessarily or even presumably long-deferred) end.
Merriam-Webster summarizes the usage dispute:
Since the mid-19th century, many grammarians have drawn a distinction between continual and continuous. Continual should only mean “occurring at regular intervals,” they insist, whereas continuous should be used to mean “continuing without interruption.” This distinction overlooks the fact that continual is the older word and was used with both meanings for centuries before continuous appeared on the scene. Today, continual is the more likely of the two to mean “recurring,” but it also continues to be used, as it has been since the 14th century, with the meaning “continuing without interruption.”
The words’ relative frequencies over the last two hundred years are shown in a curve from the Google Ngram Viewer (click to enlarge):
It’s worth comparing with data from the smaller but more reliable Corpus of Historical American English (COHA). Continual:
Collocations and synonyms
We can get an idea of the adjectives’ favoured contexts by examining their collocates – the words they tend to occur beside. To do this, I looked at a few of Mark Davies’ contemporary and historical language corpora.
iWeb shows continual collocating mostly with improvement, basis, growth, process, use, learning, development, service, improvements, professional, support, change, and monitoring. It shows continuous collocating with improvement, integration, shooting, use, delivery, process, learning, flow, monitoring, operation, service, basis, and development.
Lots of overlap, then, and both words occur most commonly in a fairly narrow contextual range. GloWbE shows similar patterns.
COHA’s older data set shows continual collocating mostly with change, struggle, state, source, presence, stream, effort, motion, changes, danger, agitation, fear, succession, conflict, flow, and dread. It shows continuous collocating with line, action, stream, flow, process, series, succession, improvement, roar, operation, service, spectrum, session, development, supply, and motion.
Again there’s overlap, but notice that continuous, reflecting its scientific origins, has traditionally tended to partner words that are more physical and less abstract. We can also see, underlined, some negative words collocating frequently with continual, hence its connotations of annoyance reported in some dictionaries.
Because of their lack of symmetry, Burchfield recommends using synonyms outside of technical contexts: ‘recurrent or intermittent instead of continual, and unbroken, uninterrupted, or incessant instead of continuous.’ Larry Trask, in Mind the Gaffe, offers similar advice. But this will be heeded only by some of those concerned about the potential for confusion, which most people aren’t.
Trask calls the distinction ‘subtle but clear’, but it’s not always obvious which word best fits a given activity. Would we say a busy receptionist handles phone calls non-stop despite lulls? That a week of heavy rain is incessant even if there are breaks? Many would. It can depend, as Merriam-Webster’s Pocket Guide to English Usage says, ‘on whether a given writer perceives the activity as interrupted or not’.
The prescriptive rule and its status
When two words are so similar in sense, there arises in certain personality types an urge to separate them categorically, the better to order the language. With continual/continuous the Merriam-Webster Dictionary of English Usage (MWDEU) dates this trend to Elizabeth Jane Whately’s Collection of English Synonyms (1851), which assigned the ‘recurring’ sense to continual and ‘uninterrupted’ to continuous.
Prescriptive guides have upheld this distinction ever since. Here’s the influential Chicago Manual of Style in the US: ‘What is continual is intermittent or frequently repeated. What is continuous never stops—it remains constant or uninterrupted.’ And its UK equivalent, the New Oxford Style Manual: ‘continual: constantly or frequently occurring; continuous: without interruption, unbroken’.
This neat division is echoed continuously – continually, if you prefer – in dozens of usage books. Laurence Urdang’s Dictionary of Differences says the words are ‘discriminated in good style’, yet one of his examples – ‘continuous hiccuping’ – is debatable per the rule he advocates. Webster’s Compact Dictionary of Synonyms uses watery images to delineate the words’ scope:
Continual implies a close prolonged succession or recurrence <continual showers the whole weekend>. Continuous usually implies an uninterrupted flow or spatial extension <the continuous roar of the falls>.
The Columbia Guide to Standard American English illustrates the distinction with gunfire:
A continual annoyance is one that goes on repeatedly, as in Continual bursts of gunfire kept us awake all night. Presumably it was intermittent, with silences between bursts. Continuous works in either time or space and means “unending, without interruption.” Continuous gunfire would not cease even for a moment.
Take a moment, if you like, to process the characterization of all-night gunfire as an ‘annoyance’.
Garner’s Modern English Usage refers to the words as ‘frequently confused’. But given their long-entangled usage, can we assume confusion in people who don’t observe the rule in its strictest form? Jane Austen wrote about ‘a continual state of Inelegance’; R. L. Stevenson, in Treasure Island, described a breeze sprinkling the floor with ‘a continual rain of fine sand’.
Both examples are from MWDEU, which says continual is ‘more likely’ than continuous to be used for ‘repetition of something that may be interrupted’, and that continuous, unlike continual, ‘is used of continuity in space’ – Samuel Johnson noted this too – and retains technical uses in maths, biology, etc. But both words, MWDEU says, are routinely used in similar contexts:
Because it has a much broader spectrum of application, continuous is the more common word. Yet, continual has been used since the 14th century in its primary sense of “continuing indefinitely in time without interruption” and is still used in that sense. Continuous is more recent in this application, having become established in it only in the 1830s, but it is very frequently so used in current English. Continual is the word most often chosen when the meaning is “recurring”. It is possible for you to follow Miss Whately’s distinction or one of its modern versions, and many good writers do so. Others equally good do not, however.
The adverbs are less complicated, but also less neat than sticklers would prefer. Mirroring the adjectives, continually arrived in the early 14thC, continuously in the late 17thC. In the intervening centuries, continually did its double duty as both ‘incessantly, constantly’ and, less strictly (OED), ‘with frequent repetition’.
Burchfield picks up the story:
The arrival of continuously signalled a fairly general restriction of continually to the less strict meaning. From the 18C. onward, continuously has tended to be the more dominant word of the two, esp. when the unbroken nature of a process (in space or time) or set of events (e.g. unbroken terms of office) is being stressed.
But he notes that good writers often use the adverbs more or less interchangeably, depending on the degree to which continuity is being emphasized. At the moment they occur at roughly the same frequency (at least in the Google Books corpus, to which caveats apply):
MWDEU finds it ‘very difficult to discern a marked difference’. But its files show that continuously ‘does all the technical work’ and is used ‘when something continues in space rather than time’, to stress that something happens steadily rather than episodically, and to stress ‘an unbroken succession of discrete time periods’.
Continually, per MWDEU, is used especially ‘when something continues to exist or happen, with or without interruptions, for an indefinite period’, or ‘when repetition is emphasized’, as in Muriel Spark’s reference to rereading certain works continually. It says its examples
show uses of continually and continuously in which a fastidious writer can make a rational choice. But these do not represent all usage. Many times the words are used as if they were interchangeable.
Per MWDEU, it’s usually continuously that encroaches where continually ‘would have worked as well and might have been expected’, rather than the other way around. This, it says, probably owes to the steady increase in use of continuous and continuously ‘rather than the result of confusion, indifference, or a failure of standards’.
Others would attribute the encroachment to precisely those elements. You get to decide where your sympathies lie.
If you want to apply the distinction but you struggle to remember it, try a mnemonic. Continuous ends in an ‘s’, which is like a river flowing without interruption. Continual forms the adverb continually, whose ‘ll’ shows the intermittent repetition invoked. Garner suggests “one uninterrupted sequence” for continuous.
In formal contexts it’s worth at least being aware of the distinction sometimes preferred, and you may take satisfaction in observing it. In everyday usage it’s unlikely to matter: you can trust your hunch or sprachgefühl, mindful that a few listeners or readers will notice, or you can apply the rule, mindful that many won’t.
Many ‘confusables’ are less messy than continual and continuous. I’ve written previously about complement and compliment, defuse and diffuse, discreet and discrete, flaunt and flout, militate and mitigate, peak, peek, and pique, pore and pour, principal and principle, refute and reject, stationary and stationery, and who’s and whose.