Ah, wikipedia. Could not find my Strunk and White.
An apostrophe is used by some writers to form a plural for
abbreviations and symbols where adding just s rather than ’s would
leave things ambiguous or inelegant. While British English did
formerly endorse the use of such apostrophes after numbers and dates,
this usage has now largely been superseded. Some specific cases:
In the US, it is generally accepted to use apostrophes to show
plurals of single lower-case letters, such as mind your p’s and q’s.
However, this is far from universally accepted practice, and instead
of writing “ps and qs” which may look slightly confusing, many
editors recommend “Ps and Qs” or “Ps and Qs.”
For the plural of abbreviations, an apostrophe is widely regarded as
incorrect, so CDs is preferable to CD’s.
For groups of years, the apostrophe at the end cannot be regarded as
necessary, since there is no possibility of misreading. For this
reason, many authorities prefer 1960s to 1960’s (although the latter
is a common Americanism[1]), and 90s or ’90s to ’90’s.
The apostrophe is sometimes used in forming the plural of numbers
(for example, 1000’s of years); however, as with groups of years, it
is unnecessary: there is no possibility of misreading. Most sources
are against this usage.
Finally, a few sources [2] accept its use in an alternative spelling
of the plurals of a very few short words, such as do, ex, yes, no,
which become do’s, ex’s, etc. In each case, dos, exes, yesses and
noes would be preferred by most authorities. Nevertheless, many
writers are still inclined to use such an apostrophe when the word is
thought to look awkward or unusual without one.
Winsor
Long Beach, CA
USA
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