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An easy way to add meta keywords to your posts and pages. Just put the keywords in the text field of each post or page and they will appear when the post or page is displayed.
<h3>Keyword Research</h3>
<a href="https://localruler.com" rel="nofollow ugc">Get keywords at localruler.com</a>

Local Ruler Meta Keywords Manager

An easy way to add meta keywords to your posts and pages.

Philipp Speck

This plugin will make it possible to use tags as <code>meta keywords</code>. Under <code>Pages</code> you will find a new meta box called <code>Keywords</code>. 
By activation the plugin registrates a separate taxonomy which is independent from post tags. 
Today’s search engines don’t use <code>meta keywords</code> as a criteria at all, but who knows for what they will be good in future 😉 
For me tagging is necessary to bring pages in relation and gives me the possibility to save my tags redundant in the database.
Features
<ul>
<li>Use post tags as <code>meta keywords</code></li>
<li>Registrates own <code>taxnomy</code> for pages</li>
<li>No coding needed</li>
<li>Language support</li>
</ul>
Related Links
<ul>
<li><a href="http://www.typomedia.org/wordpress/plugins/wordpress-meta-keywords/" title="Homepage of WordPress Meta Keywords" rel="nofollow ugc">Homepage</a></li>
<li><a href="https://wordpress.org/extend/plugins/wordpress-meta-keywords/changelog/" title="Changelog for WordPress Meta Keywords" rel="ugc">Changelog</a></li>
<li><a href="https://wordpress.org/extend/plugins/wordpress-meta-keywords/faq/" title="FAQ for WordPress Meta Keywords" rel="ugc">Questions</a></li>
</ul>
Localization
<ul>
<li>German translation by <a href="http://www.destio.de/" title="Designstudio, Philipp Speck" rel="nofollow ugc">Designstudio, Philipp Speck</a></li>
<li>Slovak translation by Branco Radenovich</li>
</ul>
Note: 
Before installing or upgrading new plugins, please backup your database first!

WordPress Meta Keywords

This plugin gives you the ability to set Meta Keywords in WordPress. Under pages it will registrate a separate taxonomy for keywords.

Local Ruler Meta Keywords Manager vs WordPress Meta Keywords