Unfortunately, this question, while understandable, isn’t very helpful when trying to understand plagiarism. Plagiarism is about citation, not duplication, and it’s possible to use passages of text verbatim without plagiarism as long as it is quoted and cited correctly.
Often times, the question is asked with reference to paraphrasing: When is a work changed enough so that it doesn’t need to be quoted but can be considered original?
However, once again, the question isn’t very helpful. Paraphrasing isn’t about changing the words of the original source, but about writing the ideas in your own words. If you’re paraphrasing correctly, none of the original verbiage should be present.
In short, when writing an essay, there’s no time in which you should be altering the words of a source in hopes of making them original. Anything copied from the source should be quoted and cited accordingly and all other writing should be as original as possible.
But that doesn’t mean that any matching text is automatically a case of plagiarism. In fact, a 0% matching score is often as suspicious as a high percentage matching.
This is because even if two essays are written completely independently, there’s a good chance that they will have some overlapping text between them. This is especially true if they are written on the same topic.
This includes common phrases, relevant expressions, cliches and even some passages that may only have one way to say it properly.
In short, matching text, by itself, is not an indication of plagiarism. To determine if matching text is plagiarism a human needs to evaluate that the text was copied inappropriately and not cited correctly.
Plagiarism detection services are powerful tools, but they are meant to aid human evaluation, not replace it.
That being said, it’s also possible to plagiarize without copying text at all. One can plagiarize ideas and information just as easily as they can text. So having information and ideas that are not common knowledge but not cited can also be plagiarism.
What that means is that, even if your words are original, your work can still be a plagiarism if your ideas and information are not and are not cited.
While this may seem frightening and like there are infinite ways to fall into the “trap” of plagiarism, the solution is actually very simple: Citation.
As long as you do your own writing and cite your information as well as any quotes you use, you should be fine.
After all, the golden rule of avoiding plagiarism is this: When in doubt, cite!
If you follow the golden rule, you should be fine.