The Plural of RPM?

Subject: The Plural of RPM?
From: Geoff Hart <ghart -at- videotron -dot- ca>
To: tom -dot- green -at- iwon -dot- com, TECHWR-L <techwr-l -at- lists -dot- techwr-l -dot- com>
Date: Tue, 02 May 2006 15:15:17 -0400

Tom Green wondered: <<I'm having a back-and-forth with an engineer about how to write more than one RPM.>>

By default, acronyms can inherently be either singular or plural, and you'll see some acronyms with an added "s" to make the distinction clear: one PC but several PCs, and so on. But units of measurement receive special treatment: the unit is always expressed as a singular acronym or abbreviation (i.e., no terminal "s"), and the only indication of whether one or more of those units is being discussed comes from the number that precedes "RPM".

You can explain this to your engineer by example: you'll never see "the distance was 5 ms [or: 5 m's]" (where m = metre) in any scientific or engineering document. The SI standard for units is that they do not take an "s" in any case; "s" is reserved for the unit of time known as the second. (See, for instance:

Moreover, for "revolutions per minute", it's exceedingly rare to discuss only a single (1) RPM. By default, this means the unit is almost always plural.

Where it's necessary to distinguish between singular and plural and you can't use numbers to do so (i.e., when the acronym is not a unit of measurement), the most common style is to use a combination of articles ("a" for singular) and verb forms (singular versus plural) to make the meaning clear. But you will occasionally see plurals formed using the same rules used for any other English noun. Thus:

<<She wants to write, "RPM's." I freaked and said, "No, that would be saying, it's the Rotation Per Minute's problem.">>

The most common style nowadays is to simply append the "s" with no apostrophe to create a plural. However, the apostrophe has a long and legimate history of use for forming plurals, as in phrases such as "mind your P's and Q's" and "a list of do's and don'ts". In such usages, there is no confusion that the meaning is intended to be possessive, and that's why the apostrophe is allowed.

This stylistic device was traditionally adopted to avoid problems with words such as "as": does this mean "more than one letter A", or the word "as"? The Grammatical Powers That Be decided by consensus that using the apostrophe to mean something other than the possessive was justified by the increased clarity of meaning. You can undoubtedly find this specific format in any good comprehensive style guide, but it's not relevant justification for adding an "s" to a unit of measurement such as RPM. The genre conventions overrule the grammatical in this case. (Live by the rules of your audience!)

<<I also hate to apply the possessive to an inanimate object or a company for example, "IBM's net worth." Is there a rule about that, or is that just me?>>

It's not just you, but avoiding this form of possessive contradicts many centuries of English usage: nowhere does it say that only living things can possess something, and English would be a horribly stilted language if you couldn't say "the mountain's shadow", "the perfume's aroma", and so on. Although it's true that one must actually think briefly about whether it's incorrect to personify the inanimate, the only situation when that personification must reasonably be avoided is when it misleads the reader. (You see this problem all the time in discussing evolution... even scientists have been led astray.)

Where the result is not misleading, there's no good justification for avoiding the apostrophe-s, and you won't find any rule against this in most modern style guides. (I'm tempted to say _any_ modern guide, but I suspect that some of the more anal-retentive guides out there, such as APA, probably do proscribe this usage. <g>)

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Geoff Hart ghart -at- videotron -dot- ca
(try geoffhart -at- mac -dot- com if you don't get a reply)
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The Plural of RPM: From: tom -dot- green -at- iwon -dot- com

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