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There are numerous ways to say how things are similar, though they are not always interchangeable
THE VARIABILITY OF SIMILARITY-NAMING
Discovering and naming similarities is a major way of thinking analytically, especially during classification (see 162. Ways of Writing about Categories). As a result, similarities feature quite heavily in both academic and professional writing (see 94. Essay Instruction Words and 115. Surveying Numerical Data).
English has numerous ways of indicating a similarity. Some just say that a similarity exists:
(a) Greek cuisine is similar to Lebanese.
More often, however, there is a need to also say what the similarity is – for example that the two cuisines in (a) both use olive oil and white cheese. Once again there are various ways of doing so.
Another cause of linguistic variety is whether or not the writer expects the reader to be familiar with one of the two similar ideas. This is not surprising, since familiarity expectations quite regularly affect language choice in English (see 156. Mentioning What the Reader Knows Already). Consider the following similarity statement:
(b) Coal stations pollute, like motor vehicles.
The underlined words here indicate that the reader already knows about motor vehicle pollution, so that the sentence is primarily asserting the polluting nature of coal stations, motor vehicles being mentioned only to make this main message clearer. On the other hand, if the two similar ideas are both expected to be unfamiliar to the reader, the language chosen to express them might look like this:
(c) Coal stations pollute, and so do motor vehicles.
Even more variability of similarity-naming comes from the fact that it can be done in not just a single sentence in ways like those above, but also in multiple sentences, just as is the case in other key writing functions like example-giving, expressing consequences, listing, specifying and naming exceptions.
This post surveys the main ways of expressing a similarity in English. For a similar survey of difference language, see 216. Indicating Differences.
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NAMING A SIMILARITY IN A SINGLE SENTENCE
1. Similarities to a Familiar Idea
The word(s) indicating this kind of similarity in a single sentence may be a verb, adjective, preposition or conjunction. Common verbs are RESEMBLE and MIRROR:
(d) Coal stations resemble motor vehicles in polluting.
One could also say can be likened to. Note the use of in before the similarity (polluting). It is usually followed by either a verb in the gerund (-ing) form, as here, or that and a subject and ordinary verb (…in that they pollute – see 153. Conjunction Uses of “that”). In informal contexts one might see because instead of that (see 82. Common Errors in Making Comparisons, #7).
Adjectives indicating a similarity to a familiar idea can be made by adding -like to the familiar idea (provided it is just a single word), e.g. truck-like (see 106. Word-Like Suffixes and 163. Ways of Naming Properties). Other adjectives are synonyms of the preposition like (see similar etc. below).
The main preposition for comparing something with a familiar idea is like (see 56. Comparing with “Like” and “Unlike”), used either alone or after a “degree” adverb (a little, exactly, just, much, quite, somewhat, very). The familiar idea will be the noun after like. Surrounding commas may be necessary, depending on meaning. Consider (b) again:
(b) Coal stations pollute, like motor vehicles.
This says what coal stations do, and likens it to the familiar behaviour of motor vehicles. Without the comma, however, the focus would shift from what coal stations do to how they do it: their way of polluting resembles that of motor vehicles (see 53. “As”, “Like” and “Such As”). Grammatically, like then relates just to the verb before it (pollute), not everything before.
In (b), with or without commas, the like phrase is adverb-like. This allows like to be paraphrased by comparably to, similarly to and in the same way as. Elsewhere, however, like phrases may be adjective-like, describing a previous noun:
(e) The illness caused by the new virus was like influenza.
The noun being described here (illness) is separated from like by was. This use of like has such synonyms as akin to, analogous to, comparable to and similar to. All can be weakened with approximately, fairly, quite or roughly. The same as is also a synonym if used after almost, approximately or roughly (it cannot follow fairly or quite: see 231. Confusions of Similar Structures 3, #2). For further aspects of the same, see 87. “Same As” versus “Same That”.
Another preposition that can help show a similarity to something familiar is the second as in as… as… statements:
(f) Cheetahs run as fast as cars on a freeway.
The first as here is an adverb quantifying fast (see 194. Adverbs that Say How Much). The second as introduces the noun cars, implying it to be familiar to the reader. Thus, (f) is about the speed of cheetahs, and uses the speed of cars on freeways to make it clear. Comparisons using as… as… are typically needed when the similarity – fast in (f) – is expressed with either an adverb or an adjective. For more on the adjective use, see 159. Exotic Grammar Structures 2, #5.
One other way of showing how a new idea resembles a familiar one in the same sentence is by placing the latter after a conjunction. This is only possible if the familiar idea contains a verb, since conjunctions usually need one. Here is an example containing the conjunction just as:
(g) Just as water travels to the lowest possible level, so heat transfers to cooler substances.
In this case, just as introduces the first of the two verbs in its sentence and, with so before the other one, is part of a “double” conjunction (see 64. Double Conjunctions). Just as can also go between the two verbs, but then has no partner so.
The verb after just as in (g) (travels) is different from the other one in the sentence (transfers). This is a common use of just as, though as alone is often possible too. Just as can also be used when the two verbs are the same:
(h) Aeroplanes are fuelled by kerosene just as oil lamps are.
As this shows, the main verb (are fuelled) when repeated after as is abbreviated. Repetition of multi-word verbs is done by means of their first word alone (are above), while single-word ones become DO (without so: see 212. Special Uses of “Do” 1), unless they are BE, which is itself repeated.
If a sentence like (h) has as or (informally) like instead of just as, there is sometimes a comma in front, depending on meaning, as with the prepositional use of like in (b).
One other similarity-showing conjunction, as if (+ past tense), indicates an exact resemblance (see 191. Exotic Grammar Structures 3, #3).
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2. Similarities to an Unfamiliar Idea
In sentence (c) above, the words and so do imply that pollution by motor vehicles is new information. The verb do is an abbreviated repetition of the previous verb, determined in the same way as repetitions in sentences like (h). After and, various other expressions are also possible instead of the slightly informal so do…, including …act(s) similarly and …do(es) the same.
Similarities of this kind involving negative verbs need neither instead of so:
(i) Helium does not easily form compounds and neither does argon.
One alternative to and so is as. To distinguish it from the earlier-mentioned use introducing a familiar idea, the following noun and abbreviated verb have to be mentioned in reverse order, like in direct questions: as do motor vehicles (see 159. Exotic Grammar Structures 2, #1).
Another formal possibility with two unfamiliar ideas is both. It may refer back to them as a pronoun or adjective-like “determiner” (see 271. Tricky Grammar Contrasts 3, #2), or forward as a conjunction partnering and (see 64 Double Conjunctions):
(j) Both coal stations and motor vehicles pollute.
Neither… nor… likewise indicates a similarity between two unfamiliar negatives.
Two unfamiliar ideas can also go without both before a similarity verb like …resemble each other, are similar, are (almost) the same or are alike. The nature of the similarity is usually specified with in:
(k) Coal stations and motor vehicles resemble each other in causing pollution.
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SIMILARITY-NAMING IN MULTIPLE SENTENCES
There are two main ways of indicating a similarity with two or more sentences. In one, two ideas are named and said to be similar in the first sentence, and the nature of the similarity is identified afterwards:
(l) COAL STATIONS resemble MOTOR VEHICLES. They (both) produce harmful gases.
The first sentence here allows the usual alternatives to resemble – mirror, act like, are like and are akin (or comparable or similar) to. An alternative is to begin There is a similarity (or a resemblance) between… (see 161. Special Uses of “There” Sentences, #2). One advantage of this is that the similarity can be characterised with an adjective like major or interesting.
If a list of similarities is to be given, the first sentence can show this by ending in…ways, the dots representing an exact or vague number word (see 122. Signpost words in Multi-Sentence Listing and 96. Avoiding Untruths 2). An alternative to ways is respects (but not aspects – see 81. Tricky Word Contrasts 2, #4).
The second sentence should not normally have any special wording to show how it is linked to the first (see 117. Restating Generalizations More Precisely). Its subject may be they both, suggesting the behaviour of both similar ideas is new to the reader, or just it/they, referring just to the first-mentioned idea and suggesting only its behaviour is new.
The other main type of multi-sentence similarity-describing names one of the two similar ideas in the first sentence and the other in the second. There are various ways of doing it:
(m) MOTOR VEHICLES produce harmful gases. They thus resemble COAL STATIONS. (SAME BEHAVIOURS/FAMILIAR SECOND POINT)
(n) MOTOR VEHICLES produce harmful gases. COAL STATIONS are similar/do the same. (SAME BEHAVIOURS/UNFAMILIAR SECOND POINT)
(o) HEAT transfers to cooler substances. It resembles WATER travelling to the lowest possible level. (DIFFERENT BEHAVIOURS/FAMILIAR SECOND POINT)
(p) BIRDS are optimised for flight. Similarly, AIRCRAFT are designed with flying in mind. (DIFFERENT BEHAVIOURS/UNFAMILIAR SECOND POINT)
“Same behaviours” here means that the behaviour named in the first sentence – produce harmful gases in (m) and (n) – is also implied in the second sentence (coal stations do it too).
It is noticeable here that familiar second points begin with a pronoun matching the main noun of the first sentence – they in (m) repeats motor vehicles; it in (o) repeats heat. Also noticeable is the fact that only (m) allows a consequence connector (thus, therefore, hence, consequently), presumably because of the specific features involved (same behaviours/ familiar second point).
In contrast, the connector similarly (or synonyms such as likewise and in the same way) seems possible only in combinations like (p) (different verbs/ unfamiliar second point). Note that in a similar vein (see the end of 241. Some Common Figurative Phrases) has a different (topic-introducing) use.
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