Which is grammatically correct: "first and last name" or "first and last names"?

What is your first and last name? Answer: X and Y

What are your first and last names? Answer: X and Y

What are your first, middle and last names? Answer: X, Y, and Z

All equally valid and grammatically correct. In spoken use as a previously posted Ngram shows spoken and written can vary. Who cares if you are in a suspect line-up. But if writing a corporate biggie, always best to strive for correct grammar.

both work. although was often, first and family/christian name/s. again. the plural actually acts for the covering of MIDDLE name/s, if applicable.

I find both grammatically incorrect, instead, you should ask "What is your first and last name?"

I don't believe you'd ever hear someone say "What are your first and last name?" That seems wholly incorrect to me. B definitely seems more valid than A, though if possible, I'd stay away from using are.

A. What is your first and last name?

D. What are your first and last names?

Isn't A asking one person for her first and last name while D would be used when asking a group of people for each person's first and last name? For example, if a cop wanted the names of everyone in a group who was caught smoking, she would ask, "What are your first and last names?" But, if she was only interested in the teenager who had supplied the cigarettes, she would ask, "What is your first and last name?"

A. What is your first and last name?

B. What are your first and last names?

Since "what is" agrees with "first and last name" and "what are" agrees with "first and last names" your question is really just between name and names.

"Which form is more commonly used by the people" is a question very different from "which is grammatically correct", although frequent and widespread use of a phrase, construction or idiom confers its own validity quite independent of grammatical correctness, which is determined by the formal "rules" of the language.

According to Google Ngrams, which is a powerful and credible indicator of how the language was/is (being) used, if not necessarily of strict grammatical correctness, both forms of the phrase, "first and last name" and "first and last names" have surprisingly near-identical pattern of use -- one is not used a lot more than the other^[1]-- and both have been used with steeply increasing frequency since 1975:

Source: Google Ngrams

That makes both forms equally valid from a descriptive perspective and you can use "name" or "names" without changing the meaning.

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^[1]Update: OP @AleksandrH has reported in a comment that a google search for each of the 2 sentences in question was done as recommended by @MaxB and "What is your first and last name?" has about 250k more hits. It is worth noting that Google Ngrams searches books while Google search actually records how many people used the exact phrase as found in all online sources.

[So that is how people use it @AleksandrH: the overwhelming majority, I should think! 250k is a huge margin of difference... Thanks for the information. Thanks too to @MaxB for the insightful search suggestion. Grammatically correct is something else, but it is hardly relevant here, because the People have spoken: and the winner is "What is your first and last name?"]

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As for strict grammatical correctness, only "what are your first and last names" is correct because they are 2 names, which is plural (except when a person has just one name in total, which is considered a mononym and not usually categorized as "first and last name"), but the way people speak is often more common-sensical than grammatical!

Both are correct. As one man said, such is the beauteous efflorescence of the English language. With the rise of bureaucracies and the filling in of forms, we can imagine how "first and last name" came to be viewed as a collective noun.

Here's the logic: You only have one legal, official name, and it is composed of parts. You're asking about two of those parts: the first, and the last. So to be grammatically correct, the question should therefore be posed:

• What is your name (first and last)?

If you desire more complete information, ask:

• What is your full name?

The question does not ask about nicknames or aliases, so there is no context for plurality.

I'll use MaxB's examples and labels:

A. What is your first and last name?

B. What is your first and last names?

C. What are your first and last name?

D. What are your first and last names?

I agree with MaxB that "D" is grammatical (as ma8 points out, "D" is obviously correct if we are asking about multiple first and last names, for example in a question addressed to multiple people) and "B" is ungrammatical.

"your first and last name"

It certainly seems to be the case that people say things like "first and last name", using the singular noun "name" rather than the plural noun "names". This trend is clearer if we use another possessive pronoun, since "your" can be plural or singular:

The Google Ngram Viewer indicates that in 2008, "his first and last name" was a bit more common than "his first and last names" (although they look close), "my first and last name" was more common than "my first and last names", and "her first and last name" was more common than "her first and last names".

C. What are your first and last name?

I disagree with MaxB's answer in that I don't think it's correct to characterize "C" as "ungrammatical". As MaxB points out, it can be interpreted "your first name and last name" with ellipsis of the first "name".

This structure seems to be used in published works: here are a few examples taken from Google Books of "a first and last name" being used as a plural noun phrase:

• score if a first and last name are mentioned, or if only a last name is mentioned, but not if only a first name is used.

  ("Scoring manual for psychological stances towards the environment", by Abigail J. Stewart, in Motivation and Personality: Handbook of Thematic Content Analysis, 1992)

• Her first and last name are given, while he is known only by title and last name

  ("Child-Animal Relationships in Twain and Stanley Hall", by Joan Menefee, in Enterprising Youth: Social Values and Acculturation in Nineteenth-Century American Children's Literature, 2008)

And of course you can find many examples online with a simple Google search.

Note that people say things like "both my first and last name", which I think would have to take plural verb agreement.

However, constructions like this do seem to be somewhat controversial, judging by the answers to the following question: "Both the first and the last [plural]" vs. "both the first and the last [singular]"

D. What are your first and last names? (said to a single person)

It does seem that it may be more common to use "first and last names" rather than "first and last name" when there is a following verb that takes plural agreement. The Google Ngram Viewer gives the following frequencies for "first and last name(s)" followed by "are" or "is" (as shown by the unexpected frequency of "first and last names is", there seem to be some false hits along the lines of "the importance of first and last names is", "compound names consisting of first and last names are", "the letters in his first and last name are", but I don't think those affect the result):

I don't agree with Hugh's answer because I think the frequencies of "is first and last" and "are first and last" are unlikely to be similar to the frequencies of "is your first and last name(s)" and "are your first and last name(s)".

A. What is your first and last name?

I'm not sure if I agree with MaxB's statement that "A is the right choice if you only need one full name", or Nick Uva's suggestion that "first and last name" can be interpreted as a "collective noun".

The only sense in which I'm sure it would be grammatical for me is unlikely to be encountered in practice: "The name that you had from the start and that you will continue to have till the end" (as in the sentence "That was the first and last time my mother yelled at me").

I would be more inclined to interpret it as somehow being an elliptical version of a question like "What is your first name, and what is your last name?"

I found the following bits of dialogue from novels that may help illustrate my point:

• “Emergency operator, what is your name and your problem?”

  (Gargoyles in the Mist, by Jonathan Saville, 2011)

• “Tell me child what is your name and age?”

  (In the Moonlight, by Sarah Helton, 2011)

Neither of these seems to have been edited very well, if at all, but I think that's irrelevant to the point I'm trying to make: that people do in fact sometimes use "What is X and Y" in situations where it seems unlikely to me that the coordinated noun phrases "X and Y" can be interpreted as referring to some single thing.

That said, I guess it does seem possible that this is a situation where "your first [name] and last name" is treated as an "X and Y" coordination that takes singular agreement, as discussed in Singular verb after two noun phrases joined by "and" that can be thought of as a single thing.

There is another, even more relevant question that I upvoted, but that unfortunately has no answer: "What is your name and height" or "What are your name and height"