Below the edit box is a Show preview button. Pressing this will show you what the article will look like without actually saving it. It is strongly recommended that you use this before saving. This allows you to check what the page will look like first and to check that you haven't made any errors. It also prevents the need for multiple saves. Saving the same article several times in quick succession makes it harder for people to check what changed, and clogs up the page history.
If you have been making small edits to the article, it is helpful to save the page before moving, breaking apart, or combining sections. The difference display is not good at showing paragraph moves, and will not display the small edits clearly if sections are then moved around.
If you are logged in, you can set your preferences to show the preview before the edit box instead of after it. If you select this option, the preview will be displayed above the edit box when you click the "Show preview" button while editing a page. See Help:Preferences.
In the case of automatic conversion of wikitext the preview shows the rendering, while the button "show changes" shows the new wikitext.
Using show preview also prevents your login timing out, which would mean your edits would not be attributed to your user name. Clicking preview regularly will restart the timer, so you remain logged in. Also, if you are no longer logged in, showing a preview will make you aware of this so you can log in and save your work after that. You can check this by seeing if your user name appears in the upper-right corner when you press preview. If you are not logged in, open Special:Userlogin in a separate window (right-click "Log in" and hit "Open link in new window" or similar). Log in, then go back to your editing and hit "preview" again to make sure all is well. Note that this will clear the minor edit and watch checkboxes, so make sure they're set appropriately again before you finally save the page.
When using this feature during a major edit, consider copying the content into your clipboard. The Wikipedia servers sometimes have hiccups, and you could lose your work.
Typically, the noinclude-part of a template page contains examples including or substituting the template. Changes in the working of the template (i.e. changes outside the noinclude-part) are not yet effective in these examples in preview and, in the case of substitution, in "show changes".
Pressing "preview" also produces a preview of the edit summary, so e.g. links can be checked.
Quick preview/AJAX preview
You can get an AJAX preview button which usually works faster than waiting for the whole page to reload using the standard preview button.
There are numerous scripts for this purpose, but there is not yet an official version on the English Wikipedia.
- User:Js/ajaxPreview, adds a preview button above the edit textarea.
Only for Commons
On Commons there is an official quick preview feature. You can use commons:MediaWiki talk:Gadget-QPreview.js. This script can be activated using the Gadgets tab on the Preferences menu.
- Wikipedia:That's why we preview, kids - an essay