Why don't we pluralize "foot" in measurements?

For example, to answer the question, "How tall are you?" valid answers include:

• Five feet.
• Five foot three.
• Five feet, three inches.

Why the discrepancy between feet and foot, seemingly only in the second case.

This question is inspired by this question: "Forty foot" or "forty feet"?

edit: I do not believe the answer to this is related to the other question. The explanation for the other question is because of how adjective modifiers work. My question is a very different case, unrelated to adjectives. My observation is that I am asking about a particular exception case which applies only when "foot" is followed by a number which is assumed to be inches. That's extremely specific, and I doubt that the etymology has any relation to why we leave adjective modifiers singular.

Solution 1:

There are many such cases of using singular forms for plural meaning, not only in English, but also in German, and possibly other Germanic languages, or even non-Germanic languages. For example, you order "drei Bier" instead of "drei Biere", and in a football match, there are "elf Mann" on the pitch, rather than "elf Männer". (The plural forms are not strictly incorrect, just less common.)

In general, the "why" question doesn't have a satisfactory answer for this kind of linguistic fact; don't search for logic and reason where there's only history and convention.

Ah, and of course you also say "drei Fuß" in German (on the rare occasion you'd be referring to pre-metric measures), not "drei Füße", which, in this case, would have to be qualified as wrong, just as in English, I guess.

Solution 2:

While, to my ear, the distinction you mention sounds right, it is not quite supported by what we see in published literature.

For example, both of the following are attested as replies to How tall are you?, and I'm not sure one is significantly more frequent than the other:

[1] a. I'm about five f̲o̲o̲t̲ ten. (sources)
      b. I'm just five f̲e̲e̲t̲ two. (sources)

Similarly, both of the following are attested as well:

[2] a. I am five f̲o̲o̲t̲ two inches tall. (sources)
      b. I am five f̲e̲e̲t̲ three inches tall. (sources)

In CGEL, for example, we find both of the following:

[3] a. Our room is twenty f̲e̲e̲t̲ by thirty f̲e̲e̲t̲ (p. 655)
      b. My other table is six f̲o̲o̲t̲ by four. (p. 693)

Just in case one is tempted to think it significant that in [3a] the unit appears twice while in [3b] the second appearance it is ellipted, note that e.g. the following are attested:

[4] a. Heizer's Double Negative is a 50 f̲o̲o̲t̲ by 30 f̲o̲o̲t̲ by 1,500 f̲o̲o̲t̲ double cut in Virgin River                  Mesa, Nevada. (sources)
      b. Here is likewise a court-yard 40 f̲e̲e̲t̲ by 37. (sources)

I conclude that the distinction under discussion is regional or idiolectical---that it doesn't really exist in Standard English. It seems that in Standard English, both foot and feet are acceptable in the present context, though in other contexts it may not be so (e.g. it is definitely a ten-f̲o̲o̲t̲ pole, not a ^*a ten-f̲e̲e̲t̲ pole; see here).