Discussion Forum—A Way with Words, a fun radio show and podcast about language

First, welcome.

Abbreviations of all types are problematic. The rules applied to conventional words simply don't apply. For example, in traditional footnotes, the abbreviations for page and line, p. and l., are made plural by doubling, so pages and lines are pp. and ll. Example: pp. 199-206 (This book sounds very interesting till the "concerning …" part.)

One could argue that even if the last word represents the main noun, you shouldn't add the s to form the plural, since it too would be abbreviated the same way. However, I have never heard anyone make that argument. In many cases, the abbreviation takes on a life of its own apart from the words it represents. So we have VIPs, DVDs, DAs, PhDs, VPs, MVPs, YMCAs, CSIs along with RPMs, WACs, and RBIs.

At one point people said US of A. Now it's just USA. What is the plural of Very Important Person? Is it Very Important Persons? Very Important People? No matter. Even if People has no s, the plural of VIP is VIPs.

To address your specific question, I would propose, however, that RBI has come to stand for "Run Batted In" in the singular as in "He just got another RBI"; we don't say "He just added one to his RBI." So redundancy of the plural is no longer an issue, and we are left with the question of a "proper" way to represent the plural of an abbreviation, as addressed above. In this case, I think that RBIs is a much better alternative to RBI, with the plural Runs still abbreviated as R unchanged, or to RsBI, with the pluralizing s placed after the initial of the appropriate word Runs.

Don't get me started on the question of the article a or an with RBI.

In the mainframe computer world, computer speed is measured in Millions of Instructions Per Second or MIPS. Only nowadays, inevitably, "MIP" has become a noun with the plural "MIPs"; a machine is said to have 10 MIPs, or to be a 10-MIP machine.

I'll add "MPs" (members of Parliament), "ETAs" (estimated times of arrival) and MCs (masters of ceremony) to the list.

I know I have heard MPGs for gas milage and PSIs for tire pressure but I, personally, would not pluralize these.

It seems to me that many of the problematic abbreviations, those whose plural is formed by changing some word other than the last word, are scientific or statistical: e.g. MPG, RPM, RBI. My hypothesis is that the ones that tend to be expressed as integers, especially those that represent countable objects, appear to have a strong pull to pluralize as they come into common use. Those that display their ratio nature, tend to have fractional parts, and represent measurable objects, resist pluralization more successfully.

For example, I see bathroom fixtures rated with GPF (Gallons per flush) with ratings of 0.8, 2.2, or 3.5. When looking online, I do see some significant reference to GPFs, but I can discount many (not all) of these, because they are referring to the fixtures as a class by their GPF rating, rather than to the metrical unit itself. e.g. '... these clog less than 1.6 GPFs made within the last three years."