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Schema appears to have two plural forms that are both valid: schemata and schemas. Are they completely interchangeable; or are there any guidelines on which one is appropriate for particular contexts?

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I thought I'd asked for a comment from whoever downvoted, but either my request was removed or I just imagined posting it, so - please do give me feedback if there is any reason to downvote this. –  Alok Aug 10 '12 at 2:13
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Thank you @araucaria for having started a bounty here :) –  Elberich Schneider Dec 29 '14 at 21:26

2 Answers 2

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The Greek schemata is mostly found in academic writing (e.g. experimental schemata), whereas the anglicized schemas is used freely in both technical and more general discourse (e.g. database access schemas).

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This is exactly correct. –  tchrist Aug 10 '12 at 1:26
    
The "competition" for the bounty has just stiffened :) –  ScotM Dec 30 '14 at 18:47
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@ScotM I just saw nothing wrong with my database schemata ;) –  Jon Hanna Dec 30 '14 at 18:54

When a word comes into English from another language (or as a coinage using parts from another language) with a countable noun sense, there are two possible approaches to pluralising; to also borrow the plural (hence criteria for criterion, and bacteria for bacterium) or using the normal -s and similar productions of English plurals (pianos or the longer pianofortes rather than the Italian piani and pianoforti).

There's no logical rule that says we can't say "The musicians got sick because the piani were covered with bacteriums", but language exists in a space of rough consensus, and the consensus is that piani and bacteriums are wrong, while pianos and bacteria are correct, and we keep using them because that's what everyone else does.

While many words go strongly one way or another, there are several words in English that have both a loan-word plural from the same source as the singular and one produced in the normal -s production for English plurals: radius with radii and radiuses, cactus with cacti and cactuses, virtuoso with virtuosi and virtuosos.*

As with things like which spelling became the accepted spelling, whether a coinage or slang word becomes considered "a real world" and so on, it's hard to predict which will happen until a point of consensus is reached at which point one form becomes "just right" and another "just wrong", or alternatively two forms continue to compete for a long time (consider lit and lighted competing for 400 years to be the past of light as an example of the same thing happening with a different case where there can be a comparable split).

There are a few things that can encourage it to go one way or another. The more the average English-speaker who uses the word is to have at least a passing knowledge of the source language, the more likely it is to survive in English (hence Latin and Greek plurals survive more due to their former educational importance). A language that doesn't often have plural forms will often not have that lack of plural form continue into English (hence "two ninjas" is more likely than "two ninja"). Words that are commonly used in the plural will more often keep them (hence criteria and bacteria survive easily) or even overthrow the singular (data is often used as an uncountable or even singular, displacing datum).

Contrasting to that, if a word is relatively obscure, then so will its plural be, and people will produce an -s plural lacking any other choice.

As a general rule, usages will become more and more "normal" over time, and that includes words with foreign plurals tending to become -s plurals.

This ngram would suggest that this has been happening with schemas/schemata: Bearing the fallibility of ngrams in mind, it would still seem that schemata was the more popular for some time, but schemas overtook recently.

Since people use language as those around them use it, use in a particular community may go strongly one way or another, and it could be that this is at play here. The term schema comes up often in terms of databases today, and many people using the word in that context would be otherwise unfamiliar with the word (and hence more likely to use a productive than an imported plural). Considering the following, they may be the community that has pushed schemas past schemata:

At the same time though, part of the SQL standard is called "SQL/Schemata" and some databases have a view or table called "Schemata" (a table in MySQL and a view in SQL Server and PostgreSQL), which is likely to keep it at least known about in that community.

The opposite pressure used to exist in the XML Schema standard as its documentation seemed to favour schemata once, though that community seems to have moved to schemas. Still, both exist in current use, including this announcement that uses one in one sentence, and the other in the next!

Are they completely interchangeable;

All the above is a way of arriving at "yes, they are completely interchangeable". While one of the other might be more common in certain contexts, there are none where one is so massively more popular as to make the other seem wrong, or even strange.

or are there any guidelines on which one is appropriate for particular contexts?

Not really, for that reason. If you're worried then use what others around you are using, but if you just like the look and sound of one more (I personally prefer schemata) then by all means do as you will.


*There are two other possibilities. One is that people may apply the wrong foreign plural rule, e.g. producing octopi. Another is that joke plurals may force foreign forms onto plurals that didn't generally use them, and later other people may not get the joke, leading some people to think that virii and penii are alternatives to, or even more "learned" than, viruses and penises when those were just jokes.

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Thanks for the humerus ending! No one mentioned conflation with the more normal scheme? –  ScotM Dec 30 '14 at 18:45
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@ScotM do you think that conflation occurs and has an impact? It's plural is clearly schemes which isn't that close to either schemas or schemata and while it comes from σχῆμα it does so via the Latin schema (plural schemae or schemæ) so nor is someone with the origin strong in their mind going to pluralise it similar to schemata. –  Jon Hanna Dec 30 '14 at 18:53
    
As a student of Greek, that distinction is apparent to me, but Everyman does not think as deeply about his words. The conflation is an unsubstantiated theory, that I wish I had time to explore! –  ScotM Dec 30 '14 at 18:57
    
The student of Greek knows better. The non-student of Greek either "just knows" that the plural is schemata or just sticks an -s on the end. The student of English etymology, akin with the student of Latin, knows that scheme came to us via mediæval Latin and so not directly analogous anyway. –  Jon Hanna Dec 30 '14 at 18:59
    
@ScotM Incidentally, I have genuinely come across people who didn't realise that virii was a joke (and in fairness, since it was a joke among techies and techies were their source of information on computer viruses, why shouldn't they take it in earnest?), and I'm not too sure whether one person honestly thought penii was correct or was good at dead-pan. –  Jon Hanna Dec 30 '14 at 19:02

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