Help:Diff

Links to diffs are found on various special pages other than page histories, including the User contributions page which every user has, Recent changes, Related changes, and watchlists.

Using the history of a page, it is possible to generate a diff between any two versions – select the left radio button for the older version, the right radio button for the newer version, and click Compare selected revisions.

While editing a page, the Show changes button displays the changes you are about to make. The diff is also shown during an edit conflict.

This example shows the top of the diff page (some of the links are dummies). The older version is shown on the left, the newer version on the right.

Unchanged text is dark grey on light grey (only parts before and after changed text are shown). Paragraphs which have changed are black on white, with an orange border on the old version side and a blue border on the new version side. Inserted and removed text is highlighted with the border color and bolded. Where whole paragraphs have been removed or inserted, no highlighting or bolding is applied and the other side is blank.

The links "Revision as of date" link to each version. If the newer version is the latest one, the link will be to the usual page and the wording will be "Current revision (date)" instead.

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Wikipedia