There is often confusion about whether to use in or of (and sometimes from, or none of the above) in the content or title of a Wikipedia article (or category, portal, template, etc.). The advice below generally applies to all of this, including in-article text, though editorial conflict and confusion about it most often arises in title discussions.

Use in for the non-intrinsic or non-endemic:

• History of the Jews in Russia
• the Rocky Mountains in the United States (if we had an article on that subtopic)
• Elves in fiction

Use of for the intrinsic or endemic:

• Category:Ports and harbours of Canada
• Indigenous peoples of the Americas
• Template:Languages of Middle-earth

(This kind of distinction has been discussed in detail on Wikipedia since at least 2007.)

Let's look at these examples in more detail. Jewish people may be born in Russia and move to the UK or to Botswana, or wherever. "Jews of Russia" would imply an endemic group that arose there independently, and is uniquely of that place.

The Rocky Mountains run through Canada and the United States (and Northern Mexico, depending on definitions); it's a single range that is in all these places incidentally, and is of North America endemically. "Rocky Mountains of the United States" and "Rocky Mountains of Canada" would imply two unrelated ranges with coincidentally the same name, each separately of each of those countries.

Lots of fiction (and pre-fiction folklore) has elves; they're found in those works but are of the broad Northern European folk tradition. Tolkien's inventions are the languages, races (including elves of various unique sorts particular to his oeuvre), places, and other elements of his Middle-earth fiction series, and they originated therein (though not, of course, without inspiration by prior materials). To make the distinction clearer: lots and lots of things are not of those fictional works but found in them, such as castles and horses and hats, friendship and warfare and perseverance.

Canada's ports are established and maintained by the government of Canada [not just "in Canada", by the way] and its provincial and more local subdivisions, so they are intrinsic, ergo they are of Canada, not just in it. They didn't creep into the country from the US and Russia and Greenland.

Indigenous peoples and other ethnic groups exist all over the world, but each is endemic to a particular region and thus is of it. However, they are not always mostly within a particular geological landmass or a modern political boundary, especially after centuries of migration. When a specific one is not (aside from individuals moving around – we care here about mass settlement), use in – thus "Jews in Russia".

Another use of of in Wikipedia article titles is for the unusual case that an article on the characters in a work of fiction is not a list article (which would be a "List of Title Here characters" page); such non-list characters articles are titled in the form "Characters of Title Here". Do not use in or from in these constructions.

Switching from in to of or vice versa can produce a shift in meaning sometimes, not just a lack of sensible meaning. For example, the topic "languages in Middle-earth" would imply a topic of the authorial treatment of languages in the series, in the voices of various fictional characters, perhaps including critical study of how the author used differing cadence and formality levels to imply cultural and class differences, etc. (While that might be a valid topic for someone to write a literature paper about, it's likely not an encyclopedic topic for a Wikipedia article, and definitely not under such a title.) Meanwhile, "languages of Middle-earth" implies that the topic is the invented languages themselves – their grammar, syntax, vocabulary, and writing systems – and we do have a whole category of encyclopedic material on this.

Another example: the Yanomami are a small tribal culture native to Brazil and Venezuela. "The Yanomami in Brazil" refers to the segment of this population that is within the political boundaries of that modern country. "The Yanomami of Brazil" implies a meaningfully definable distinction between that segment and the segment who live in Venezuela, but no such distinction actually exists at that invisible jungle boundary which is just a surveyed line on a map – except in culturally irrelevant senses (e.g., the fact that the Brazilian Yanomami get counted, roughly, in the Brazilian census, and Brazilian regulations that affect anthropological, medical, and other expeditions to these tribes may differ from Venezuelan ones, a distinction that is not about the Yanomami themselves). Collectively, the two arbitrary segments of this singular culture can be referred to as "the Yanomami of Brazil and Venezuela" because they are autochthonous to an ecological zone within that two-country region.

Do not confuse or mislead our readers with sloppy wording. Using "the Yanomami in Brazil and Venezuela" implies they are not exclusively endemic to those countries and that there are also Yanomami populations in, say, nearby Colombia and Uruguay, which isn't true. Contrast this with the Jews example; there are populations of Jews in various countries, owing to various mass migrations from ancient to modern times. This is like "the Yanomami in Brazil", which is simply a narrower case where "various countries" with such populations has a total count of two.

Avoid splitting hairs obsessively, however. All the native peoples of the Americas can be said to be of the Americas or the Western Hemisphere in general – you won't find populations of them in Botswana or New Zealand. Technically, a few of the northernmost of them have ranged a little into Siberia (part of Russian Asia) and into Greenland (which may or may not be classified as part of North America, depending on definition). In a broad context, they can still be included in "native peoples of the Americas" (same goes with alternative phrasing such as "indigenous groups", "traditional cultures", etc.). However, using "native peoples in the Americas" in an attempt to be over-precise and nit-picky, to exclude the small number that aren't in North through South America, is apt to confuse readers into thinking you mean or at least include groups who are sometimes defined as ethnic, who are from other places, and who have relocated to the Western Hemisphere, such as the Amish or the Patagonian Welsh. Remember that our goal is to communicate to the world with clarity, not to play hyper-technical logic games with ourselves.

In a context where the distinction matters, one can write of the Yupik in Alaska and the Yupik in Siberia more specifically, for example. If reliable anthropological sources have identified crucial differences between those two populations, then of could also be used in a context discussing those differences, but it should probably not be done otherwise, because the reader may be mislead into thinking these populations are distinct in every way and in every context, which is definitely not true; their culture, like other cultures (Scottish, Arabian, Japanese, etc.), intergrades across their entire contiguous range.

There can also be confusion about from. This word is used in the titles of categories and lists of individual people to indicate their place of origin or strong association, e.g. Category:People from Melbourne. It should not be used as a substitute for in or of in broader types of categories or articles, especially if the resulting construction seems informal or is ambiguous. Avoid: "List of characters from Star Wars". (Did they emigrate from the Star Wars universe after getting refugee status in the Star Trek franchise?) The convention for such pages is the "List of Title Here characters" format.

In and on can sometimes conflict in usage. When writing about an island, use whatever is actually idiomatic in the majority of mainstream English for the particular place in question, as judged from major, contemporary, reliable sources (not local publications – Wikipedia doesn't care how residents of the place talk, since the encyclopedia is not written in 1,000 micro-dialects). It will almost always resolve to using "on" for small islands; using "in" for larger ones; using "in" for independent countries that happen to be (or to be within) islands; and also using "in" for island groups. Thus, "on Santa Catalina Island" but "in Madagascar". It has nothing to do with population level, or with inhabited versus uninhabited status; Iceland was unpopulated for much of human history, but we can still write that "the environmental effects of vulcanism in Iceland during the Pleistocene is well understood." When writing about an island as a jurisdiction, in is the normal usage, regardless of size, though it may be clearer writing to use the full name of the jurisdiction (e.g.: "The legal system in the Bailiwick of Jersey is a mixture of Norman, English, and modern French traditions"). The same island, when being written about as a human-geography place, not a legal entity, might be on or in, depending on size ("Several survivors of the shipwreck remained on Jersey, and their descendants still live there today").

For most categorization purposes, use in, not on, for any construction that would use in for a category about a non-island (Category:Football in Jamaica); use from for any category that would use that word for a non-island (Category:People from Okinawa Island). Wikipedia has a strong interest in category names forming consistent patterns (otherwise people are apt to create duplicate categories).

Some island-unrelated in and on usage patterns are simply a matter of differences in major varieties of English. E.g., one might write "The family lived on Dundas Street in Toronto until 2001" in North American English, but "The family lived in Dundas Street in Edinburgh until 2001" in British English. (Neither of these actually make much sense if taken literally, of course; people live beside streets.) This sort of usage difference is not likely to affect categories or article titles, just running prose.

There are many cases where no such wording should be used at all and a prepositive modifier (such as an adjective) should be used instead. For example, we have an article at List of Middle-earth peoples; it should not be renamed "List of peoples of Middle-earth" (arguably logical but unnecessarily wordy) or "List of peoples in Middle-earth" (logically incorrect on two different levels: it is not a real place so no one is really in it, and its fictional peoples are of that fictional place and series of works of fiction, they didn't fictionally move there from Narnia or get borrowed by Tolkien from C. S. Lewis's Narnia stories). The awkward "List of peoples from Middle-earth" would be out of the question, suggesting real ethnic groups who live today in Indonesia or France or where ever but moved there from Middle-earth. This example was chosen carefully, because fossils of a real proto-human subspecies, Homo floresiensis, were discovered in Indonesia in 2003, and have been nicknamed "hobbits", the name of a race of fictional Middle-earth people from Tolkien's stories. We cannot trust that school children reading Wikipedia fully understand the exact dividing line between fictional and real-world things that share the same name, nor can we assume that all learners of English as a second language are certain of the definitional limits of unfamiliar terms in our language. Millions of readers in both categories use the English Wikipedia every single day, so keep them in mind.

As of 2018, we have a large number of articles misusing in when they should use of, and vice versa, and this situation is even messier in the category structure of the site. This has been cleaned up a tiny bit at a time, often with single-page moves. It can and should be expedited with mass requested moves of articles (with the {{Rm}} template) and categories for renaming nominations (with the {{CfR}} template). This is best done on the scale of broad meta-topics, e.g. moving all "national monuments in" articles and categories to "national monuments of".^[a]