Wikipedia:Common knowledge

A frequent justification in casual conversation is that a certain fact is "common knowledge". It often turns out that most people don't actually share this knowledge. Even claims that are widely believed often turn out to be anywhere from only mostly true to the complete opposite of what is actually the case.

Wikipedia editors are strongly encouraged to find reliable sources to support their edits, and to cite them. Citing sources when your edit is challenged by another editor is Wikipedia policy, and any unsourced edits may be removed. For more information, see No original research and Verifiability.

There are some claims that many Wikipedians find acceptable to report as fact, without citing any outside sources. This guideline seeks to define when it's a bad idea to do that.

Acceptable examples of common knowledge
''Remember: What appears to be common knowledge for some Wikipedians may be unknown to many from other cultures and backgrounds. For a different viewpoint, see WP:NOTBLUE.''
 * Known time and date relating information ("There are seven days in a week.")
 * Well-known historical facts ("Julius Caesar was a Roman.")
 * Geographic pieces of information easily verified by a non-specialized map ("Dallas is in Texas.")
 * Plain sight observations that can be made from public property ("A tall spire sits atop the Empire State Building.")
 * Mathematical or logical truisms ("1 + 1 = 2")
 * Universally-accepted everyday orders that are taught in early elementary school ("A comes before B in the English alphabet." or "January comes before February in the Gregorian calendar.")

When to add citations for claims
Certain kinds of claims should most definitely not be left to common knowledge without citations:
 * Controversial claims
 * Facts about which Wikipedians themselves disagree cannot form a rough consensus.
 * Claims in areas of fact or opinion about which there is known to be controversy. This includes political and religious ideas.
 * For a sampling of controversial topics, see List of controversial issues.
 * Untested facts or arguments
 * Original research that presents reports based on your own experience, or your own ideas, theories, or arguments, even when these are based on established facts, are not allowed, according to Wikipedia policy.
 * Facts that cannot be confirmed by Wikipedians other than the original claimant.
 * Technical knowledge
 * Claims that something is a scientific fact. Acceptable scientific theories are published and peer reviewed.
 * Medical claims. There are many pitfalls, false leads, and confusing details and countervailing factors in medicine. It's also very important to report only accurate information.  Even though Wikipedia readers are cautioned not to use the encyclopedia instead of visiting a physician, we wouldn't want anything bad to happen to someone because of an inaccuracy here.  There are plenty of written sources that are more authoritative than the average Wikipedian; see here for advice on finding them.
 * Claims that something is legal or illegal. Legislation, regulations, and case law are all published. Also, just citing a law that seems to make a specific instance of something illegal is not always enough.  There may be other laws which override the cited law in the situation under consideration, and there are many details of the application of the law which complicate matters.  Also, what is illegal varies by jurisdiction (for example, there are few drugs that are illegal everywhere).
 * Anything where a PhD (or other advanced training) is required in the field to be able to evaluate truth and consistency with the consensus view; for example, black hole thermodynamics.
 * Historical facts. An account by a professional historian, or if none is available, a contemporary written account. In the latter case, such a primary source should be interpreted with caution.
 * Linguistics: Language data beyond the smallest local communities, for example regional or national data on word use. Language variation is rich within a language. The data compiled by lexicographers and linguists quite often shows usage different from any one individual's everyday experience.
 * Indirect knowledge
 * Hearsay: If you heard or read something somewhere, you must be able to cite a reliable published or broadcast source. Don't play the telephone game.
 * Anything the reporting Wikipedians don't have direct personal experience with. Most of us don't have personal experience with space travel, or the atomic bombing of Hiroshima.  But many of us have experienced popular music, know our local geography, and are familiar with the meanings of words within our local communities, although, as always, if your edit is challenged, no matter how convinced you are that you're right, you must cite a reliable published source.
 * See Reliable sources for more information on these topics. Also see No original research, which is policy.

Should I believe what other editors say?
You should evaluate the testimony of Wikipedians as you would any other primary source. Keep in mind that it can be rude to simply tell someone "I don't believe you" or "I think you are lying" or "You are so biased; no one should believe anything you say." Many people honestly hold mistaken opinions, and no one likes having their beliefs rejected. Many people also don't realize that the experiences of others are different from their own until other people share them, but are perfectly willing to be enlightened if it's done in a civil fashion.

The most diplomatic thing might be for someone to affirmatively say "I don't think that's correct, and here's some evidence from outside sources or my own experience which don't seem to match up with what you wrote".

If you are thinking "that sounds fishy", but don't have any evidence to support your skepticism, say so. Many readers will have the same doubts. If you have a specific reason for doubt, definitely mention it. If not, you can simply ask some questions derived from Reliable sources. "That sounds odd to me. Can anyone else verify that?" or "If we took a poll of experts in the field, would they all agree with this?" or "Is there a published source we can cite for this?" or "Is there anyone who is not {a supporter of the cause, a member of the cult, etc.} who could confirm this or offer another perspective?"

See Wikiquette.

Has it always been that way?
Some facts may be so-called "common knowledge" today, but weren't known in the past or weren't obvious. It's a good idea for there to be some explanation of how these facts were discovered, how they have since been confirmed. For example, that the giant ball of fire in the sky is called the sun is an easily verified fact: all you have to do is check a dictionary. The fact that the Earth revolves around the sun is also a fact, but it's far from obvious from simple observation. A link to the history of this scientific discovery would be excellent documentation.

It can be a good idea to explain how things came to be the way they are. The fact that the letter [A] is the first letter of the alphabet is an easily verified fact, which can be looked up in a dictionary. A link to a linguistic reference that explains the origins of the alphabet would be excellent documentation, although a link to confirmation from a dictionary would suffice.

Weasel words
When reporting claims and opinions, so-called "weasel words" tend to crop up, like "some believe", and "others claim", which should always be avoided. Replace the weasel words with names of people, institutions, or publications, and cite the source of your claim. See Avoid weasel words.