Let's say we have the numeral "12,345". Why is it pronounced as "twelve thousand three hundred and forty five" as opposed to "twelve thousands three hundreds and forty five". Are 's's omitted while pronouncing numerals to make the whole thing more succinct and easier to pronounce?
Singular vs. Plural – Why No Plural ‘S’ After ‘Hundred’ or ‘Thousand’
numbersreading-aloudsingular-vs-plural
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Consider a sentence where the apples are the subject.
One hundred and one apple rolls down the street. That sounds pretty strange. A disagreement with the verb would be even worse.
It's more than one apple, so it should be plural.
I am a native speaker with a careful ear. From my experience, I can tell you that when the millennium turned from 19xx to 20xx, we said "two thousand" plus the remainder throughout the aughts (01, 02, ..., 09). To use the "twenty" construction would have required acknowledging the zero digit: "twenty oh-eight, twenty-oh-nine" or "twenty-aught-seven" etc. Those were still heard a lot, however, as we were all new to the millennium and its numbering.
When it hit 2010, we started mostly saying "twenty-ten, twenty-eleven, twenty-twelve," etc., but it was not uncommon to hear "two thousand thirteen." This somewhat ambiguous pattern will likely continue throughout the teens.
You may be reasonably sure that once we hit the twenties, the "twenty-something" construction will overwhelm the "two thousand" one because "twenty" is still easier to say than "two thousand," and by that time it will also be more felicitous to use because "twenty twenty" has already made inroads into the general ear due to its appearance in popular culture as a standard of vision, etc.
Addendum
Just to add to an appreciation of the discomfort we feel with the awkwardness of "two thousand", consider the year 2000 itself. It's almost invariably referred to using the confection "the year 2000." Compare uses of "the year 2000" vs, say, uses of "the year 2012" in the Wikipedia articles on those years: "the year 2000" is referenced five times in the three-paragraph article (four in text, once in footnotes), while "the year 2012" in its article is given only once (all other times referring to that year simply as "2012").
Note that the millennial year was sometimes referred to as "Y2K"—although that term is blurred because Y2K could refer to the year itself or (more often, I think) to the turning of the millennium as a concept.
Now, given that we had primed our ears for "two thousand" by using "the year 2000," and that for thirty-odd years we had the example of the very popular Stanley Kubrick film 2001: A Space Odyssey, which everyone I ever heard talk about the film referred to as Two Thousand One and not Twenty-Oh-One, it seems reasonable to suppose that these uses laid a "two thousand" groove from which it was somewhat difficult to extricate our tongues.
Note that I am not saying it was impossible: people still did use "twenty-oh" while the rest of us muddled along doggedly using "two thousand" until 2010 (when, again, not all of us switched). All I am saying is that the millennium shook up the way we pronounce years, and we are slowly returning to the old pattern, the way ripples in a pond slowly die out after a rock has been tossed in.
Order will be restored. Be patient.
Addendum
Last night I was watching a news program and heard the announcer refer to events that happened in "two thousand nine and twenty ten" ... which was interesting.
Best Answer
Because in English "twelve thousand" is not interpreted as 12 occurrences of 1000, as in 12 cars, but rather as one occurrence of 12000. Why it's like this, is a matter of speculation.
Languages are different and I guess some languages view 200 as a plural, as two occurrences of 100. English and Dutch do not, and French, as 200_success points out, swings both ways: 200 being deux cents, but 201 being deux cent un.