In informatics I often come across a sentence of the type (with or without commas):

"The data is read from, and written to, disk by the operating system."

I don't know whether the use of commas is correct in this case. I've read a few resources online but found nothing conclusive. One of the first hits I got when searching was this one and I was thinking the sentence fits with

Rule #3: Use a comma to set off nonessential elements.

A nonessential element is a word, phrase, or clause that is not needed to complete a sentence. In other words, it can be removed and the sentence still makes sense and is grammatically correct. If removing the element changes the meaning of the sentence, it is essential. Nonessential elements need to be offset with commas, both before and after.

However, both reading/writing are essential operations---from a functional perspective---when it comes to a hard disk.

What would be the correct approach, commas or no commas?

  • 1
    In your case, drop the commas.
    – Lawrence
    Commented Nov 6, 2017 at 2:46
  • 1
    Note that the presence of commas doesn't necessarily imply "and written to" is nonessential. It can have the effect of highlighting the phrase, depending on which words are accented. Dropping the commas gives reading and writing similar emphasis (to each other).
    – Lawrence
    Commented Nov 6, 2017 at 3:06

1 Answer 1


This is an RNR (right node raising) construction, and the most popular theory of its structure is due to Paul Postal. That structure puts major constituent breaks after each constituent apparently conjoined by "and", in the places you've put commas. In any event, it is often possible to make breaks in intonation in these positions. Intonation breaks are typically indicated by commas in English orthography, but there is no well defined answer to whether they would be in any given example. Probably, the longer and more complicated are the partial constituents connected by "and" (or some other coordinate conjunction), the more likely it is that commas would be used.


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