Correct usage of “parallel” versus “in parallel” versus “parallelly”

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.


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.

Source : Link , Question Author : Jos van Egmond , Answer Author : F’x

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