There are numerous ways to name a similarity between two nouns
THE VARIABILITY OF SIMILARITY-NAMING
Discovering and naming similarities is a major way of thinking analytically. This means, hardly surprisingly, that it features heavily in both academic and professional writing (see 94. Essay Instruction Words and 115. Describing Numerical Data). Sometimes there is a need to say nothing more than that a similarity exists between two things, as in the following example from the Guinlist post 82. Common Errors in Making Comparisons:
(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 feta cheese. English has numerous ways of doing this, and it is this more detailed kind of similarity-giving that the present post is about.
In some of these ways, one of the two similar ideas is expected to be already familiar to the reader or listener, while in others neither one is. The language used for expressing the similarity will often vary according to these expectations. Consider the following example from the post 56. Comparing with “Like” and “Unlike”:
(b) Coal stations pollute, like motor vehicles.
The underlined words here (which could also begin the sentence, with a comma after instead of before) 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 considered 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.
For other examples of the way information familiarity can affect language choice, see 156. Mentioning What is Known Already.
The variability of similarity-naming comes from the fact that the two different kinds can be placed in not just a single sentence, as above, but also in multiple sentences. And the similar ideas can be associated with the same verb, like pollute above, or with different ones. My aim here is to survey all of these possibilities. The post is thus similar to various others about meanings capable of being expressed in either one sentence or many (see, for example, 32. Expressing Consequences, 33. Complex Example-Giving, 117. Saying More Precisely What You Mean and 122. Signpost Words in Multi-Sentence Listing).
SIMILARITIES SPECIFIED WITHIN A SINGLE SENTENCE
1. Comparisons with Familiar Ideas
In (b), where only one of the two similar ideas is expected to be new to the reader, one could directly replace like with in the same way as. Alternatively, and less directly, it is possible to use a verb instead of like, as in this example:
(d) Coal stations resemble motor vehicles in polluting.
One could also say act like, can be likened to, are like, are akin (or comparable or similar) to. and mirror. Note the use of in before the identification of 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). In informal contexts one might see because instead of that (see 82. Common Errors in Making Comparisons).
An important feature of (b) is the comma before like. A very different comparison is made if it is removed:
(e) Coal stations pollute like motor vehicles.
Although like here is still suggesting familiarity, it now links more closely with the word before it (pollute) than with the whole sentence. As a result, the reference is no longer just to the fact that motor vehicles pollute, but instead to the way in which they do: coal stations are being said to create the same kind of pollution as motor vehicles. For more about this contrast, see 56. Comparing with “Like” and “Unlike”.
Another same-sentence way of showing a similarity to something familiar is with as … as … . A typical use might be:
(f) CHEETAHS can run as fast as CARS ON A FREEWAY.
Of the two compared nouns here (in capitals), the second is implied to be familiar to the reader. In other words, (g) is about the speed of cheetahs, and uses the speed of cars on freeways to make it clear. Comparisons with as … as … are typically needed when the similarity – fast in (g) – is expressed with either an adverb or an adjective. For more on the adjective use, see 159. Exotic Grammar Structures 2, #5.
In sentences (b) and (f), the two compared nouns have the same verb (pollute and run). Elsewhere, however, the verbs may be different, like this:
(g) Just as WATER TRAVELS to the lowest possible level, so HEAT TRANSFERS to cooler substances.
As this shows, the equivalent of like when there are two different verbs is just as before the familiar information. It can start the sentence, as here (in which case there must be a following so), or come in the middle (with no so at the start). More on combinations like just as … so … is in the post 64. Double Conjunctions.
2. Comparisons with Unfamiliar Ideas
Sentence (c) shows a rather informal way of naming a similarity between two new ideas. It can be generally characterised as and so + AUXILIARY VERB (DO to repeat a one-word verb; BE, HAVE or “modals” like will, should, might to repeat multi-word verbs). You can also use as instead of and so (as do motor vehicles – see 159. Exotic Grammar Structures 2, #1). More formally, one can say and … act(s) similarly (and motor vehicles act similarly) or and … do(es) the same.
When a similarity involves a negative verb, there is a need for neither instead of so:
(h) Helium does not easily form compounds and neither does argon.
In more formal writing, a similarity between two unfamiliar ideas can be expressed with both … and … (or neither … nor …):
(i) Both coal stations and motor vehicles pollute.
Alternatively, one could use the phrase resemble each other:
(j) Coal stations and motor vehicles resemble each other in causing pollution.
SIMILARITY-NAMING IN MULTIPLE SENTENCES
There seem to be two main ways of dividing a similarity statement between two or more sentences. In one, the two similar nouns and an assertion of their similarity are communicated by the first sentence, and the nature of the similarity is identified afterwards:
(k) COAL STATIONS resemble MOTOR VEHICLES. They (both) produce harmful gases.
If a list of similarities is to be given, the first sentence can show this by adding in … ways, the dots representing a number word (see 122. Signpost words in Multi-Sentence Listing). Either an exact number word like three could be used or a vague one like various (see 96. Hedging 2: Lists & Predictions). An alternative to ways is respects (but not aspects – see 81. Tricky Word Contrasts 2).
No special linking language is normally necessary in the second sentence of pairs like (k) (see 117. Saying More Precisely What You Mean). The verb in the second sentence will usually be associated with both of the similar nouns, as above. The use of both in the second sentence suggests that neither of the similar things is familiar to the reader; if one of them is familiar, both cannot be used.
The first sentence in (k) allows the usual alternatives to resemble – act like, are like and are akin (or comparable or similar) to. In addition, if neither of the two compared behaviours is expected to be familiar to the reader, the first sentence can begin There is a similarity (or resemblance) between … (a following both then being optional).
The other main type of multi-sentence similarity-naming places one of the two similar nouns in the first sentence and the other in the second. Compare the following:
(l) MOTOR VEHICLES produce harmful gases. They thus resemble COAL STATIONS. (SAME BEHAVIOURS/FAMILIAR SECOND POINT)
(m) MOTOR VEHICLES produce harmful gases. COAL STATIONS are similar/do the same. (SAME BEHAVIOURS/UNFAMILIAR SECOND POINT)
(n) HEAT transfers to cooler substances. It resembles WATER travelling to the lowest possible level. (DIFFERENT BEHAVIOURS/FAMILIAR SECOND POINT)
(o) BIRDS have a shape that is 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 (l) and (m) – 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 (l) repeats motor vehicles; it in (n) repeats heat. Also interesting is the fact that only (l) requires a consequence connector (thus, therefore, hence, consequently). There could well be a link with the specific conditions for (l) (same behaviours/familiar second point). On the other hand, the connector similarly (or synonyms such as likewise and in the same way) seems possible only in combinations like (o) (different verbs/unfamiliar second point).