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How do you correctly abbreviate that something is in "three dimensions" in formal writing? As per the title, would you write either "3D", "3d", or "3-d"?

I want to write something like:

The figure contains a 2D widget (left) and a 3D gadget (right).

Additionally, if I use the full phrase in a sentence, I gather that I write

The three-dimensional gadget is composed of independent two-dimensional widgets.

i.e. using a hyphen and the word "three". Would it also be allowed to write "3-dimensional"? Especially in a context where I want to contrast it with something else "2-dimensional" in the same (or adjacent) sentence, as the numerals stick out more?

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    It depends on the writing style of the particular journal or conference. – Stan Sep 22 '15 at 9:24
  • There's no "correct way" to abbreviate something. The entire point of abbreviations is that they don't have to be formal, and they save space and time spent vocalizing. So with this in mind, I guess either 3D or 3d are "most correct", whatever that means. – user180089 Jun 29 '16 at 16:32
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I am searching for an answer to the same question. I wouldn't use 3d, 3-d, or 3-dimensional because I have never seen them. Longman dictionary says three-D or 3-D (Longman), but Oxford also says 3D (Oxford). So I think 3D or 3-D are accepted.

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According to this Ngram, both 3d and 3D are acceptable to use. Furthermore, different dictionaries show a different form of abbreviation for "three-dimensional". Some are 3-D, some 3D, some 3d, and so on.

From this I would conclude that none of these are incorrect to use. 3-dimensional is not something that is recommended though, as shown from the results.

Ngram also shows that "three-dimensional" is used just as much as 3D or 3d. Perhaps that is the best option to consider using in formal writing, since formality doesn't have fondness for abbreviations.

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    I think you're being fooled by Ngrams. When I look at actual hits for lowercase 3d, it's often in constructions like "King George 3d". Look at the Ngram for 3-D and 3-d ... nobody uses lowercase in 3-d, so why would they for 3d? Especially when the common use of 3d predates use of the words "three dimensional". – Peter Shor Apr 21 '16 at 19:34
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"3d" means "third" and "2d" means "second" and these notations are used in legal terminology (although they are not used in British English, and normally one would just use "3rd" or "2nd").

3-D means three-dimensional. The hyphen is not eliminated when abbreviating the term. The same goes for 2-D/two-dimensional, or whatever numeral you decide to plug in.

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I was looking the distinction between 3D and 3d, and decide to check it by myself on technical literature (Google Scholar). It turns out that 3d is shown in the list, but upon checking the content of the paper, it is not 3d but 3D. In other word, use 3D for the correct one.

Update:

The link I used to search is: this

And the first item display (07/2018) would be:

"Learning spatiotemporal features with 3d convolutional networks"

But upon further opening the link to verify it, actually the paper would show "3D". This is true for all the other title with "3d", when further verify from the link, it is actually 3D.

  • Could you provide some links to instances using the lower case version? Please edit your post to include those links. :) – JJ for Transparency and Monica Jul 25 '18 at 1:56

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