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One of the key policies of Wikipedia is that all article content has to be verifiable. This means that reliable sources must be able to support the material. All quotations, any material whose verifiability has been challenged or is likely to be challenged, and contentious material (whether negative, positive, or neutral) about living persons must include an inline citation to a source that directly supports the material. This also means that Wikipedia is not the place for original work, archival findings that have not been published, or evidence from any source that has not been published.
If you are adding new content, it is your responsibility to add sourcing information along with it. Material provided without a source is significantly more likely to be removed from an article. Sometimes it will be tagged first with a "citation needed" template to give editors a chance to find and add sources, but often editors will simply remove it because they question its veracity.
This tutorial will show you how to add inline citations to articles, and also briefly explain what Wikipedia considers to be a reliable source.