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You'll now find a TL;DR for this answer, but you'll have to work for it by scrolling to the bottom of this answer to see it. JSYK, though, this answer is organized into relatively short sections containing images and formatted text designed to make it easier and more enjoyable for you to read.
Short answer to your question: No, it is not advisable to use "millares" when you really mean "millones" and vice versa (if the purpose is to avoid confusion). If you want to underestimate or exaggerate, well, that's a different story. Apart from that, many good answers from what appear to be native speakers have already chimed in on "millar" versus "millón," and in my mind, it has been sufficiently answered.
I will add that I wish you would have given actual examples with cited sources so that those attempting to answer your question could have assessed whether or not the Spanish you saw used had been used correctly. On at least one occasion I've posted a question about Spanish based off of something I had incorrectly assumed was standard usage/practice, when in reality, it wasn't.
Not everything you hear or read in Spanish is a correct usage of it. Honing your ability to evaluate and question sources is a good habit that, while never completely perfected, will serve you well in many areas of life. If ever in doubt, you can always find good resources like the Spanish Stack Exchange to help you evaluate.
My answer here, since I feel "millar vs. millón" has been sufficiently addressed, simply recaps some of what has already been written, but mainly focuses on what distinguishes "miles"
Definitions of "mil" and "millar"
Before I go any further, I want to lay down some basic definitions of these two words and I'm going to use the Diccionario de la lengua española (DLE) to do that. I'm also going to focus on just "mil" and "millar" because I think enough clarification has been given to put "millón" in a category all its own. Below is a chart of meanings straight from the DLE, but translated into English with a mix of the embedded Chrome translation tool and my knowledge of Spanish. I only focused on those meanings that provided an opportunity to compare and contrast one word with the other. If you want to see the full set, click on these: mil | millar.
Some notes on the table above:
- "mil" and "millar" are derived from different Latin words
- "mil" is used as both an adjective and a noun (and sometimes
pronoun); "millar" is only used as
- "millar" is not used as an exact figure, but "mil" is
- both can mean "big, indeterminate number," but when "mil" means this,
it is an adjective and when "millar" means this, it is a noun
- if you want to convey "a thousand years" using "millar," you must
insert a "de" in between "millar" and "años"
- if you want to convey "a thousand years" using "mil," simply write
"mil años"; no indefinite article is needed before it and no
preposition should be inserted between "mil" and "años"
- "mil" is to "millar" what "cien" is to "centenar"
Like I said, after looking at these definitions and reading through this thread, I feel as if I've got a good idea of when to use "millar" and when to use "millón." The tricky thing that remains for me still is when to use "mil" rather than "millar." And if it still isn't quite clear to me, I imagine it might not be clear to you or someone else who has wondered how these two words can be used.
A recap of previous answers
I realize that the differences between "mil" and "millar" have been touched on somewhat in this thread and I appreciate and value the comments that have been previously made, but I can see how they might confuse a student of Spanish and I'd like to help clear things up in some small way if I can. Before I add my two cents, let me recap what has been mentioned already:
... theoretically both could be used interchangeably.
... "millares" is a more learned, even a bit snobbish word ...
... I wouldn't use [millares] in everyday language, except if I wanted to give emphasis to the idea of a big number.
... "millares" is more common in written media ...
En plural ambos términos son sustantivos perfectamente sinónimos ...
... en singular «mil» es un número cardinal especial ... mientras que «millar» solo es un sustantivo, de modo que en singular no siempre son intercambiables.
Before I comment on any of the above, I should mention that I am not a native speaker of Spanish. However, I have taken some time to study these words a bit more thoroughly. After having done so, I have personally observed, through online sources, precisely what has already been claimed in this thread, with a couple of exceptions. First, I haven't noticed that "millares" is more common in written media (not nowadays, anyway) and second, I don't know that I'd consider usage of "millares" as "snobby," but I'll get to this second point in a bit. Let's focus on whether or not "millares" is more common in written media first.
Before I go any further, I have to admit that I'm not sure what, exactly, was compared in the statement "millares" is more common in written media. It certainly couldn't have meant that "millares" is more common than "miles" in written media (not in recent times anyway). In fact, I've found quite the opposite to be true. A quick Google search with a "News" filter will show that "miles" is more than 500 times more common than "millares."
But to be fair to raven (the Stack Exchange user who makes the claim that "millares" is more common in written media), perhaps what s/he meant was that if you're going to see "millares," you're more likely to see it in print (or electrons), than you are to hear it over the airwaves (or in speech/colloquial usage). Or perhaps s/he meant there was a time when "millares" was more frequent in written media than it is now. Both of those statements would be true and I'll tell you why I think that.
One of the resources I used to research this question was the Corpus de Referencia del Español Actual (CREA) and when I took a look at the results from this data bank more closely, there appeared to be some evidence that "millares" is less frequently found in speech because the ratio of "miles" to "millares" with an "Oral" filter clearly favored "miles," but much more so than it did when filtering for written media (the ratio of cases was 2.3:1 with a filter for oral sources and just 1.2:1 using filters for written sources). If you want to know more about CREA's oral data bank, click here. In addition to that, there appeared to be evidence, from more than one source [i.e., the Corpus Diacrónico del Español (CORDE) and the Google Ngram Viewer], that there was a time when "millares" was more frequent than "miles." But keep reading because I go into more detail on that in a bit.
For now, let me just say that, overall, in today's times, you will find "miles," in both writing and speech, far more often than you will "millares."
By the way, if you think that the instances of "millares" in oral sources may be references to surnames or place names, they aren't. Here is just one example from oral sources:
... las llamas funerarias de Indira Gandhi se elevaron hacia el cielo, ante la mirada de varios millares de sus conciudadanos y de más de un centenar de dignatarios extranjeros, entre los que se hallaban el presidente del Gobierno español, Felipe González, ...
— From Informe Semanal, 03/11/84, TVE 1. España.
But apart from that, the collocations I found certainly indicate that these two words — miles and millares — are very semantically similar and, therefore, are perfectly interchangeable in most situations. (Both raven and Fran have also mentioned that these two words are interchangeable.) Some examples:
Please note that one of the collocations is not in the plural — millar de dólares. You will find it in the plural — millares de dólares — but it seems to be far less common and you'll find "millar de dólares" much more frequently than "millares de dólares," in online news articles, especially those from more reputable news outlets. But having said that, note that "millar de dólares" is not always the same thing as "millares de dólares." It looks as if the former can mean "a thousand dollars" (as in "más de un millar de dólares") and the latter means "thousands of dollars."
You can see more examples by using this tool here. (If you use BOOLEAN search operators to search for more than one word at a time, the more frequent collocations will be returned first.)
As I researched this question, I encountered a couple of good teaching points along the way and already included some of them earlier on, but this would be a good place to mention them again, expressing them as I found them in a discussion thread. In one of the threads I came across, a contributor wrote:
No puedes decir "Cerca de un mil de personas" , has de decir "Cerca de un millar de personas"
to which another added:
pero si puedes decir "cerca de mil personas"
to which another wrote this in response:
no puedes decir "cerca de millar personas", tienes que decir "cerca de mil personas"
Boiled down, this means "un" must come before "millar," but it must not precede "mil" (if "mil" is followed by a noun it is modifying and this is true even if you're saying something like "more than a thousand dollars" which would be más de mil dólares rather than más de un mil dólares). And lastly, the word "millar" cannot be followed by a noun unless the noun is preceded by "de." You can read the entire thread here.
If it hasn't already been mentioned, "millares" is far less common than "miles." I found this to be true in books (as can be seen with the Google Ngram here), and Google searches, with or without filtering for "News" or "Books." According to one frequency list, "millar" is ranked #25189 and "millares," #31546, which means you are far more likely to encounter "gladiadores" (gladiators), "anticuerpos" (antibodies), and "pajaritos" (little birds) than you are "millar" or "millares."
I already provided a link to the Google Ngram and because nothing about it seemed all that remarkable (it just kind of looked like what a lot of people would expect), I didn't use it as an image in this answer, but when I pulled out "mil" and "millar," the graph suddenly got interesting:
Frequency in context
For whatever reason, "millares" was more frequent than "miles" during the 18th and 19th centuries and then, all of a sudden, "millares" took a sudden swing downward and "miles" took a sudden swing upward. I tried to see why this might be by taking a look at the CORDE. What I was able to uncover from this data bank did not reveal the reason behind this abrupt pattern change, but the pattern was mirrored somewhat. My CORDE search of "miles" and "millares" returned more instances of "millares" than "miles" (not by much, but still there were more). CREA results, on the other hand, were clearly in favor of "miles." These results were laid out earlier in this answer, but if you don't want to scroll back up, you can click here.
Regardless, after I took a look at the CORDE results side by side like this, I pasted them into a spreadsheet to use Excel's computation functions to compare percentages. You can see what I discovered by viewing the chart below. Using the first row as an example, the figure in the "millares" column indicates that 17.56% of the cases (or instances) in which "millares" is found (relevant to other instances of it) can be categorized as "prosa religiosa." The difference between this percentage and the one for "miles" (4.68%), in this same category, is 275% (almost three times as prevalent in that category). Beginning with "prosa de sociedad," the percentage in the last column indicates how much greater the percentage difference is for "miles" compared to "millares." The average of percentage difference greater for "millares," overall, is 101%; it is just 50% for "miles."
The time span for these results is 1495–1970. As has already been mentioned, frequencies for "miles" and "millares" changes at the beginning of the 20th century and from about 1920 on, "miles" is more frequent than "millares," and by 1970, "miles" is more frequent than "millares" by about 3:1 (according to Google's Ngram Viewer).
Once I was able to compare percentages, I could see some notable differences right away. For starters, "millares" did not show up at all in the category "Prosa jurídica" and "miles" did not show up at all in the category "Verso narrativo."
While that and other aspects of this comparison may be interesting, the results didn't explain the abrupt pattern change.
However, a peek at the excerpts from these documents CORDE provides did help address one of the points made in this thread — the point that was made about "millares" being, "snobbish." Even before the CORDE search, I don't know that I would have described the use of "millares" as "snobbish," but I do think it's true that you will find it tends to be used in more eloquent, or even just serious, types of writing. In other words, I don't think you're going to find it too often in a tweet or a Facebook post, but you will find it used in several different types of publications covering various types of topics, usually in a serious, matter-of-fact, and/or descriptive type of way.
Then again, the mere word "thousands" is often used in similar ways in English. Just think about the phrase "couple of dollars" and then compare it to "thousands of dollars." Just the sheer magnitude of the latter phrase alone implies something of far greater importance. Is "thousands of dollars" snobbier than "couple of dollars?" Perhaps, and maybe a fairer comparison would be "two dollars" instead of "a couple," or "a thousand" versus "a grand," even though "miles" isn't informal/colloquial/slang for "millares," but I think you get my point, which is this: Whether you're using "millar(es) de dólares" or "miles de dólares," the amount alone elevates, to some degree, what it is you're talking or writing about. And this is true whether or not you use "millares" or "miles" or whether or not you're talking about dollars or diamonds or donuts.
But now that I've seen these CORDE results, I have to confess: I don't see them as snobbish at all. In fact, if the following examples are representative of the use of "millares" in general, I don't even consider the use of this word "highbrow." Words such as "literary," or, more vaguely, "formal," would be a more accurate way to describe the uses of "millares" in these examples. Perhaps the mere use of the word is considered a snobbish choice, but, at least according to the examples I found, I don't think many would find the use of "millares" in these examples "snobby." Rather than ponder what word best describes the usage of "millares," let me just show you some of the categories and contexts "millares" tends to be found:
Examples of "millares" by category
Aun hoy, después de tantos millares de años, parece la aurora la resurrección del universo, el rejuvenecimiento de la naturaleza y el despertar de la vida ...
— From "La vida de la Virgen". Antolín López Peláez. España. Sermon. 1916.
¿Cuántos millares de hombres se traga cada dia la mar?
— From Tratado de la Tribulación. Pedro de Ribadeneira. España. 1589.
Para defender por agua la ciudad tenían millares de canoas y se ejercitaban frecuentemente en ese género de combates.
— From Historia Antigua de México. Francisco Javier Clavijero. México. 1780.
Pero en esos pocos momentos, bajo los resplandores de las lámparas que iluminan la escalera, siento que se expande voluptuosamente toda mi personalidad. Al otro día, una partecilla de esta personalidad será comunicada, con mi artículo, a millares y millares de lectores.
— From Madrid. Azorín (José Martínez Ruiz). España. 1941.
Como de niño tú no sabías existiera el tedio,
y eras todo asombro, inexperiencia, entusiasmo, afán de comprender,
así también seremos, una y mil veces, y millares más, en la fuga
eterna de otras apariencias;
cada morir será un renacer henchido de ilusiones...
— From Cantos del otro yo. Álvaro Armando Vasseur. Uruguay. 1909.
pasó por mi ventana
no sólo un ave del mar, sino millares,
y recogí las cartas que nadie lee y que ellas llevan
por las orillas del mundo, hasta perderlas.
Y entonces, en cada una leía palabras tuyas
y eran como las que yo escribo y sueño y canto,
y entonces decidí enviarte esta carta, que termino aquí
para mirar por la ventana el mundo que nos pertenece.
— From "Carta a Miguel Otero Silva en Caracas (1948)," Canto general. Pablo Neruda. Chile. 1950.
Ateniéndose a esta argumentación, que no nos toca ahora discutir, el lexicógrafo francés eligió unos dos millares de palabras, de esas que según él no se apartan nunca del concepto que representan, y a continuación de cada una fué reuniendo todos los vocablos emparentados con ella, sea por asociación de ideas, hábito de lenguaje, relación de causa a efecto, instrumento, etc.
— From Discurso de recepción ante la Real Academia Española. Nuevo concepto del diccionario de la lengua. Julio Casares. España. 1921.
Nuestra vista descubre algunos millares de estrellas entre las cuales unas brillan con vivo centelleo: estas estrellas se consideran soles como el que ilumina nuestros días. Por centenas de millones debemos contar los soles los cuales se verán rodeados como el nuestro de otros astros secundarios que son los planetas.
— From La escuela práctica: obra destinada a promover la enseñanza primaria moderna mediante ejercicios. Juan Beneham. España. 2003.
Comparing use of "millares" to use of "miles"
At this point, I could keep going through the list or find examples of "miles" to give you contrast, but I believe there's a character limit on these posts and there's only so many examples one can read before it becomes tedious. So, on that note, I will simply leave you with one more example, as much to sate my curiosity as anything else. For this last example, I'm going to try to find an example of "miles" in "verso lírico" simply because this would seem to be where contrasts in usage might be most visible. After all, who, but the poets of this world, choose words with exacting precision? And with that, here's the example I've chosen to use:
En esta época florecen los cactus de la costa. Lejos de esta región, en los contrafuertes de la cordillera andina, los cactus se elevan gigantescos, estriados y espinosos, como columnas hostiles. Los cactus de la costa son pequeños y redondos. Ahora los vi coronarse cada uno con veinte botones escarlata, como si una mano hubiera dejado allí su tributo de sangre. Pero se abrieron y frente a las grandes espumas se divisan miles de cactus encendidos por sus flores plenarias.
— From "Isla Negra," Una casa en la arena. Pablo Neruda. Chile. 1966.
Well, I don't know that Pablo Neruda agonized over whether or not to use "miles" or "millares" in the above passage. Clearly, he does use both words in his writing. But I encourage you to scroll back up and read again the earlier example I provided from Neruda. This time read it out loud and then scroll back down and read this example out loud. I'm no poet, but I think if you do that, you'll get a feel for why he used "millares" in "no sólo un ave del mar; sino millares" and "miles" in "miles de cactus." If the sound of these two passages out loud isn't enough, it may be helpful to remember what raven wrote in his/her answer — I wouldn't use [millares] in everyday language, except if I wanted to give emphasis to the idea of a big number.
Again, I don't know if Neruda gave much thought to whether or not he should use "millares" or "miles." It could be that in the first example, his intention was to emphasize the quantity and, in the second, he may have been more focused on emphasizing something else or perhaps nothing at all. Should the number of cacti be emphasized over anything else in that passage?
But as I've said, I'm not a poet nor do I have much experience in critiquing the art form the most talented among them have perfected. However, another possibility for why "miles" is used here instead of "millares" is simply that this poem, "Isla Negra," was written more than a decade after "Carta a Miguel Otero Silva en Caracas (1948)." It may be that by this time "millares" was falling out of favor and that this can even be seen in the work of poets.
Other views of "mil" and "millares"
Since I've mentioned CREA, I feel as if I'd be cheating you out of knowing about a useful tool related to it, if I didn't also mention that it has what is called an "annotated version." The interface of this version of CREA allows you several more ways to splice and dice data for a word. Rather than going into detail on that, I'll let you explore it yourself. You can link to it here. One of the useful features of this version is that it will create pie charts of your findings. These pie charts provide additional details on various distribution parameters. In other words, it shows you where you're most likely and least likely to encounter the word you've searched, geographically (by region/zone), politically (by country), temporally (by time period), and thematically or stylistically (by subject or genre of writing). To show you what this looks like, I've laid down images of the subject pie charts for "miles" and "millares."
This doesn't contradict the findings I encountered earlier using the other version of CREA (mentioned in a previous section), but you can see that the categories are different. Taking a look, specifically, at the charts above, you can see that "millares" is more likely to be found in "Novelas" than "Ciencias y tecnología" and this tendency is greater with "millares" than it is for "miles." (By the way, before you get to the pie charts, you will see tables listing the number of frequencies by category, which also may be very useful to you.)
Using the categories "Novelas" and "Ciencias y tecnología" as a check, these figures show that "millares" is twice as likely to be found in "Novelas" than "Ciencias y tecnología" whereas "miles" is only slightly more likely to be found in "Novelas" than in "Ciencias y tecnología."
You'll also find that these pie charts are quite interactive. You can click on each section of the pie chart and it will take you to a listing of the concordances for the word. This page will also show you the number of cases (or instances of the word) and the number of documents it appears in.
Again, if you'd like to explore this annotated version of CREA, click here.
One final thing to keep in mind: When "millares" is capitalized, it can also be a surname, a municipality in Spain, or an ongoing excavation site of a prehistoric town in what is now called Andalusia. In fact, if you do an image search using "millares" as your search word, a lot of images appear to be some sort of reference to this excavation site.
Don’t use “millares” when you really mean “millones.” The first means thousands, the second means millions, and Spanish speakers know the difference.
The words “miles” and “millares” are interchangeable.
It appears that from about 1920 on, “miles” has been more frequent than “millares”; before then, “millares” was more frequent.
Instances of “millares” during the last 50 years, are more likely to be present in publications rather than in speech or broadcasts.
In today’s times, “miles” is overwhelmingly more frequent than “millares”; in online news articles alone, it is 500 times more common.
The word “millares” has always been more frequent in works of religious prose, but is rarely, if ever, seen in legal prose.
In modern times, the word “millares” is more frequent in novels than in publications related to science and technology.
If you really want to emphasize the quantity of something, “millares” may be a better choice than “miles” even though they mean the same thing.
The word “millares,” when capitalized, can be a Spanish surname, municipality, or