<link rel="canonical" href="..." />
Google, Yahoo & MSN recently have introduced a new tag <link rel="canonical" href="http://www.mywebsite.com"/>
This new tag allows one webmaster to indicate under what exact URL he wants one page to be indexed, even if the same content or variations are available under different addresses. It allows search engines to eliminate "duplicate content".
ome other techics exists to eleiminate duplicate content on a website (such as 301 redirects) but it is not always possible to implement them for various reasons.
When your website have some pages with identical content, you should use this tag. It even should be used when small differences exists between the versions of the pages:
- Elimination of sessionID in the URls
- removal of the Affiliate ID
- Pages updated differently
Http & https protocol
It may well use link rel = "canonical" to for indexation of http instead of https versions of the pages.
When using canonical link tag DO NOT:
- use relative URLs
- Do not chain link rel = "canonical"
- Do not point link rel = "canonical" to a 404 error page or other error page
- Do not point link rel = "canonical" to point another domain than yours (can be done with a subdomain)
Try to always to use all the same URL for a given page.
Whenever possible, give preference to the use of 301 rather than link rel = "canonical".