The terms “common knowledge” and “public domain” often come up in discussions of plagiarism. In truth, neither term is really about plagiarism per se, but since they both relate to the proper use of outside sources, it’s important for writers to understand what they mean and how each term might apply to your work.

Common Knowledge

The term “common knowledge” refers to facts and information that are so well-known and clearly established that a writer can use them in their work without citing a specific source. 

Like plagiarism, student writers tend to think of common knowledge as a universal distinction–either a given fact is common knowledge or it isn’t. Unfortunately, as with plagiarism, the rules for what qualifies as common knowledge actually vary from one situation to the next, and thus the responsibility falls on you as a writer to understand the general guidelines for common knowledge and apply them to any given assignment or piece that you write.

Generally speaking, something qualifies as common knowledge if it meets all of the criteria below:

1) The information is purely factual and easily known to your audience.

When you include facts or information in your writing that you can reasonably assume your readers already know, or that anyone in your audience could easily verify without doing any significant research, then it’s generally acceptable to present those facts as common knowledge. 

For example, Carleton College, originally named Northfield College, was founded in 1866 by the General Conference of the Congregational Churches of Minnesota. This is a simple, objective set of facts that anyone familiar with Carleton might already know. Even if you didn’t know this, though, you can probably think of several places you could look to verify it (the most obvious being the college’s website). Since any reader could verify this fact in any number of ways, and any reasonably savvy reader could figure out on their own how to do this, you wouldn’t need to cite a specific source to use it in your writing.

By contrast, it is also an objective fact that in 1890, Carleton had three official political clubs for students: the Republican Club, the Democratic Club, and the Young Men’s Prohibition Club. However, even though this fact is probably available through multiple sources, none of them are particularly obvious or accessible to most readers (we discovered it by looking in the 1890 Algol Yearbook, which is only available through a secure login to the Carleton College Archives). Since this information is obscure and difficult to verify, you would want to cite a specific source to prove that you’d done your research and to guide your readers, should they wish to check your facts or learn more. 

2) The information is not disputed by your audience.

In order for something to qualify as common knowledge, you must be able to reasonably assume that your readers will accept it as true, without any significant qualification or skepticism. If there is likely to be any dispute or difference of opinion about a certain fact within your anticipated audience, then you would want to cite an informed and reliable source to verify that fact. This is where the definition of common knowledge begins to vary substantially from one audience to the next, because some audiences will accept certain facts uncritically that another audience might not.

For example, the international scientific community overwhelmingly agrees that the global climate is changing rapidly, and this change is primarily driven by human activity. Thus, if you were writing primarily to scientists, or even to readers who are reasonably familiar with climate science, you wouldn’t need to cite a source to assert that human induced global climate change is currently happening. The vast majority of your readers would already know and accept that fact, so it would qualify as common knowledge for that audience.

However, a portion of the American public does not accept that human induced global climate change is really happening. Thus, if you were writing to a general audience of Americans, it would be wise to cite a specific source for that fact, such as this report from the Intergovernmental Panel on Climate Change. This won’t necessarily persuade your audience that the information is true, but it will illustrate that other writers, with greater expertise in the subject than you, also believe that the information is true.

3) The information isn’t traceable to a single source.

When there is a single easily accessible origin point for a given piece of information, you should cite that source. Doing so allows you to acknowledge the intellectual work of whoever produced that information, and it demonstrates to your readers that you understand where the facts in your writing come from.

For example, in 2017 the Minneapolis/St. Paul area received a total of 32.36 inches of precipitation. There is no significant doubt or debate about this fact, and anyone who cares to look could easily find several reliable sources to verify it. However, every one of those sources would almost certainly get their information from the same place: the US National Weather Service, which is the official agency that collects and reports weather data in the United States. Since the NWS is generally considered the definitive source for US weather data, it would be wise to cite them directly, as opposed to citing a different source that uses the NWS’s data or not citing a source at all. This may mean that you need to do some additional research to find the original source of the data, but it’s work that’s worth doing. Citing the US-NWS directly recognizes their authority and signals to your readers that you know and used the best source for your information. 

Beyond these guidelines, it’s important to understand that the concept of common knowledge only applies to situations in which it’s absolutely clear to readers that you, the author, are not the original source of the facts or information in question, and therefore there is no risk that you will accidentally take credit for intellectual work you didn’t do. If this isn’t clear from the context of your writing, or if you’re at all uncertain whether something qualifies as common knowledge, you should cite your source. 

When in doubt, always cite your source!

Public Domain

Public domain is a legal term referring to documents, images, and other intellectual property that are not fully restricted by copyright. Intellectual property enters the public domain either when its copyright expires, when the copyright holder chooses to release it for public use, or in some rare cases where the law releases the work into the public domain automatically (as with much material produced by the U.S. Federal Government). Items in the public domain are “free” in the sense that you, as a writer, may access them and use them in your own work without paying fees or royalties. 

However, this absolutely does not mean that you can use words, phrases, images, or ideas from the public domain in your academic writing without citing your sources or documenting exactly how you’ve used them. 

The designation of public domain only applies to issues of copyright and intellectual property, and, as we noted on the “Rules and Principles” page, respecting intellectual property is only one reason that scholars document their sources. The more pressing reasons to document sources include honestly representing your intellectual work and acknowledging the ways you’ve benefited from the work of others. Thus, if you draw on an outside source and don’t clearly document how that source contributed to your writing, then you have committed plagiarism, even if the source is in the public domain.