Magento Community Edition, 1.9.x
Canonical Meta Tag

Some search engines penalize websites that have multiple URLs that point to the same content. The canonical meta tag tells search engines which page to index when multiple URLs have identical or very similar content. Using the canonical meta tag can improve your site ranking and aggregate pageviews. The canonical meta tag is placed in the <head> block of a product or category page. It provides a link to your preferred URL, so search engines will give it greater weight.

You can avoid being penalized by search engines by omitting the category from the URL, or by using the canonical meta tag to direct search engines to index either by product or category. As a best practice, it is recommended that you enable canonical meta tags for both categories and products.

To enable the canonical meta tag:

1. On the Admin menu, select System > Configuration. Then in the panel on the left under Catalog, select Catalog.
2. Click to expand the Search Engine Optimizations section.
3. If want search engines to index only pages that have a full category path, do the following:
a. Set Use Canonical Link Meta Tag for Categories to “Yes.”
b. Set Use Canonical Link Meta Tag for Products to “No.”
4. If you want search engines to index only product pages, do the following:
a. Set Use Canonical Link Meta Tag for Products to “Yes.”
b. Set Use Canonical Link Meta Tag for Categories to “No.”
5. When complete, click the Save Config button.