I wish to know if any of the following sentences are incorrect:

  • Using A and B parallel.
  • Using A and B in parallel.
  • Using A and B parallelly.

Now I suspect most people are going to simply recommend that I use "in parallel" since it is the most common. However, this is a question formulated to understand the underlying English theory. Apart from that, I'm mostly interested in using that theory to determine the use of parallelly and whether or not it is correct.

Now some research on my end. I've found the use of parallelly in several dictionaries:

Along with that, Wiktionary is quite clear on the use of the word. Along with some rules on how to use -ly which do not mention that the use of a word such as parallelly may be incorrect. I don't know how Wiktionary is fact checked, so I have come here instead asking the question to professionals.

Being a non-native speaker and having no background in human language apart from high school (computer languages all the way) this question and the following debate interests me greatly. The answer I am looking for probably makes a distinction between variants of English. I'll flag the post which provides the most comprehensive answer as the accepted answer.

  • 4
    Only tangentially related to your question, at best... there is an anecdote in some management book about an American Businessman saying to a Japanese one, "I see we're thinking along parallel lines" meaning, "in the same way." It turned out that Japanese took this in the sense of "Parallel lines never meet." – MickeyfAgain_BeforeExitOfSO Mar 7 '11 at 15:05
  • Some context for non-native speakers: 'parallelly' is not a common word at all. One would use 'in parallel', and that's it. 'Parallelly' sounds like a very infelicitous neologism. – Mitch Nov 9 '11 at 14:48
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    Also, I wouldn't take Wiktionary's word on anything. There is much good information there, but there's also a lot of crap... and it's not immediately obvious which is which. Feel free to disregard any Wiktionary entry that doesn't include citations. – MT_Head Nov 10 '11 at 1:41
  • I suggest this is much more simple than you think, as first evidenced by "Using A and B parallel" which clearly cannot work in standard English. “parallelly” might work and is that not different? – Robbie Goodwin Jul 16 '18 at 18:41

It is a question of grammar. You're looking for a word to qualify the way A and B are used, in the construct “to verb A and B qualifier”. This qualifier has to be an adverb (“a word or phrase that modifies or qualifies an adjective, verb, or other adverb or a word-group”).

Now, you want to express parallelism. “Parallel” itself is an adjective, as well as a noun and a verb. It is not an adverb, and as such, cannot be used in “using A and B parallel”. The natural adverb that derives from parallel is parallelly; though it's not exactly very common in general usage, it does exist and is attested in multiple (though not all) dictionaries. So, “using A and B parallelly” works.

Regarding in parallel, it so happens that it is a common phrase meaning “occurring at the same time and having some connection”. It may be more commonly used than parallelly, which is why it would feel very natural in your sentence, but both are correct.

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Your three examples are not sentences, so I will extend them with "They will be ...":

"They will be using A and B parallel" is wrong (unless there is something called "B parallel") as parallel is an adjective or noun but probably needs to be an adverb in this position. "They will be using A and B parallel to each other" looks slightly better.

The other two are strictly correct as adverbs, though care is needed to avoid using parallelly when an adjective is needed. The much more common phrase in parallel, usually following what it describes, can be used either as a adjectival phrase or as an adverbial phrase. My guess is that in parallel is more popular in part simply because it is less likely to be used wrongly.

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    "Parallelly" also rolls off the tongue about as well as a square tire. – Graph Theory Dec 19 '14 at 19:35

"Using A and B parallel" is not correct; it should be "using A and B in parallel".
"Using A and B parallely" would have the same meaning of "using A and B in parallel".

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