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What is the correct way to pluralize a proper noun like "Raspberry Pi" (a type of small, inexpensive computer)?

Would you say:

  • This project uses two Raspberry Pis
  • This project uses two Raspberry Pi's
  • This project uses two “Raspberry Pi”s

None of the options look right to me. I think the combination of being a proper noun and an unusual second word makes this tricky.

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"Raspberries Pi"? – MT_Head Jan 30 '13 at 0:34
Related (if not duplicate): What is the plural form of “iPad 2”? – Callithumpian Jan 30 '13 at 0:40
Stickler note: single digits are written as words in prose, unless those are part of the brand name etc. (thus “two of iLunch 2”). – theUg Jan 30 '13 at 2:32
@theUg - You shouldn't have to apologize for being a stickler on a website dedicated to English Language & Usage. I fixed it. :) – Nathan Long Jan 30 '13 at 3:07
@Callithumpian - Interesting. Taking Apple's approach, we'd always say "two Raspberry Pi computers." It does solve the pluralization problem, but seems a bit awkward. – Nathan Long Jan 30 '13 at 3:09
up vote 18 down vote accepted

We can approach this analytically, by authority, or by observation.


Okay, to start with our components are at least straightforward; they're both countable nouns, and the plural of raspberry is raspberries and the plural of pi is pis. We'll be using them later.

It's formed from English nouns, so there are four possible ways to treat it.

We could just not think about it, and treat it opaquely, as if Raspberry Pi were a single word. That gives us a plural of Raspberry Pis.

We could treat it as a noun adjunct, where the first noun acts as an adjective. That gives us Raspberry Pis (c.f. coffee shops).

We could threat it as a headless noun, though that seems unlikely to be correct. Anyway, this would give us Raspberry Pis.

We could treat it like a compound starting with the head, which would give us Raspberries Pi, but we'd need some strong reason to favour the first word in this manner. We can rule this out.

Of the acceptable options, since it's a pun on "Raspberry Pie", we'd favour the noun-adjunct case, but they all have the same result.

Likewise, that gives us an etymological approach: It was named to deliberately be similar to "Raspberry Pie" so we should pluralise similarly to "Raspberry Pies". That gives us Raspberry Pis.

By Authority

It was named by, and is a trademark of, The Raspberry Pi Foundation. They use the plural Raspberry Pis.

By Observation

They've been called Raspberry Pis in a variety of places.

Hence, they're Raspberry Pis.

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Unfortunately, that form brings Manneken Pis to mind. – tchrist Jan 30 '13 at 3:13
Personally I like "Raspberry Pies". Incidentally, a cluster of Raspberry Pis is known as a "Bramble" – mgb Jan 30 '13 at 3:53
Well, it certainly does now @tchrist. – terdon Jan 30 '13 at 4:26
+1 but by observation it's not hard to find Raspberry Pi's (more than Raspberry Pis), Raspberry Pies and Raspberries Pi. – Hugo Jan 30 '13 at 6:31
@Hugo I found Raspberry Pis more than Raspberry Pi's, though trying to discount repetitions and noting where I looked for one and not the other, is tricky if you're not prepared to do a detailed recorded survey (I'm not). The only "Raspberry Pies" I found in the context of Raspberry Pis seemed to be intended as a joke. – Jon Hanna Jan 30 '13 at 9:52

As a trademark, it should be an adjective, not a noun.

This project uses two Raspberry Pi devices.

Edit: The rule that a trademark should always be an adjective comes from the International Trademark Association:


In the past, I have worked for large companies for whom trademarks were important, and this is one of the rules they hammered into us.

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Trademark or no, even the creators use it as a noun (e.g. "The Raspberry Pi...") I don't know of any reason to restrict it as an adjective. – Lynn Jan 30 '13 at 5:38
As a trademark it can be any part of language at all, but is more likely to be a noun than anything else. I wouldn't even read the use above as an adjective, but as a noun adjunct. Are you saying that "a Raspberry Pi", "an iPhone", "some Jell-O", "a Big Mac", "an aspirin", "an escalator", "a zipper", "some kerosene", "an apple mac" are all wrong? – Jon Hanna Jan 30 '13 at 9:11
See my edit above. – Aric TenEyck Jan 30 '13 at 14:12
@AricTenEyck- Interesting. Clearly you have cited a valid source, and thus I have removed by downvote. But I would point out this post which I think outlines the instances where that advice is unduly prescriptive and in contradiction with how actual companies use their own trademarks. So take it with a grain of salt. – Lynn Feb 2 '13 at 22:46

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