# Singular or plural form of "digit" when referring to digits of individual numbers?

"Jim and John have strong right arms."

I believe the usage of plural form of "arm" here is correct. Now, I am confused about we should use singular or plural form of "digit" in the following sentence.

"All multiples of 10 have 0 as their units digits."

"All even numbers have even units digits."

You should use singular in this case.

There is only one units digit in each multiple, and it is 0.

Much the same can be accomplished in any complex network:

• Everyone in our branch offices has an accountant as a supervisor.
• All the people in our branch offices have an accountant as a supervisor.

The difference between the singular has and plural have is because every is singular, while all is plural (I don't make up these rules). But there is only one supervisor in each case, and it's an accountant who's the supervisor.

• Hmm. I'm not quite sure how universal the "rule" is here. "All right-handed weight-lifters have strong right arms" sounds fine to me, as so does "...have a strong right arm". But "Tom, Dick, and Harry all have a strong right arm" sounds somewhat more "iffy", and "Tom, Dick, and Harry all have a sharp mind" sounds far worse. I doubt the singular is always even "non-preferred". I actually lean towards "Tom, Dick, and Harry all have a vested interest" regardless of whether they have the same vested interest or each has a different one. Commented Feb 23, 2012 at 16:46
• @JohnLawler In that case why do we say, "Jim and John have strong right arms" even though there is only one right arm in each person? Commented Feb 23, 2012 at 17:11
• It's not the same arm. One is Jim's and the other is John's; they're both strong, but they're not identical. Commented Feb 23, 2012 at 18:21
• @JohnLawler Similarly all multiples of 10 have their own units digits. One is 10's, the other is 20's, and so on. They're all 0 but they could have been different if we were not talking about multiples of 10 only. For example, "The units digits of 15 and 20 are 5 and 0, respectively." Why can't we then say, "The units digits of 10 and 20 are 0 and 0 respectively."? If we can say this, we should also be able to say, "All multiples of 10 have 0s as their units digits." Commented Feb 24, 2012 at 8:19
• It's, as always, a question of how to rationalize what sounds right to a native speaker, using terminology and rules on which few agree. If you can do that, you should think about running for public office. Commented Feb 24, 2012 at 15:04

Aside from how this is formalized in grammar rules, you aren't comparing like with like. You'd say "John and Jim both have a hook for their right hand", not "John and Jim both have a hook for their right hands". Or you could say "John and Jim have hooks for right hands".

Similarly, you can say "All multiples of ten have zero as their units digit", or you can say "All multiples of ten have zeroes as their units digits".

So the answer to your question is that you can phrase this either with singular or with plural, but you don't mix the two.

"All even numbers have even units digits" and "All even numbers have an even units digit" are both grammatically correct. You can dispute whether one of them suggests more strongly than the other, that a number can potentially have more than one units digit. But since we know that's not the case, no such difference in meaning arises in practice for that example.

Applying the same variations to John and Jim: you can say "John and Jim have strong right arms", but you would normally avoid "John and Jim have a strong right arm". It's grammatical and might make sense in context, but the construction rather suggests they have one between them instead of one each, as you might say "John and Jim have a labrador".

You can say "All men have a strong right arm" (not true, but it parses). The implication of one arm between them could still be drawn from that, but it's much more of a stretch, it's not the "usual" meaning. So there's a difference between saying "all" and providing a list.