ALL ABOUT CANONICALS 

A canonical tag is a snippet of code included in the <head> section of a webpage.

You use it to give search engines page-level insight about original, near-duplicate & duplicate content – defining which page is the "source" of the content 

Google cares about duplicate content because it's more work for them to process - but doesn't add any real new value to search results. 

Original content should self-canonicalize.

Duplicate content should refer to the original content, thus specifying the original source.

3. Prevent URL variations from causing duplication issues 

1. Reduce the pain of  navigational issues

2. Safely syndicate content from others

Common Use Cases for Canonical Tags

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PRO TIPS

PRO TIP #1

It's a hint, not
a directive... Canonicalized pages can still be indexed if Google chooses.

PRO TIP #2

Canonical tags pass SEO equity to the referenced page, and can be used to reference any URL - on any domain

PRO TIP #3

It's always better to solve the duplicate content problem at the source. But when you can't, use the canonical.

Now For Some Precautions:

Watch Out For:

Google may not accept this format. Sometimes it does, but more often it doesn't.

Relative URLs

Watch Out For:

Pagination variations are valid. Don't canonicalize to page 1 if you want items on page 2 to be found & indexed!

Pagination

Watch Out For:

Canonicalizing to non-valid or non-indexable pages 

Accidentally, generally through URL variations (e.g. www vs non-www)

Watch Out For:

Especially if the rendered instructions are conflicting with the source code instructions.

Implementing Canonicals with JavaScript

Watch Out For:

75% of the time, you'll send conflicting signals.  Use the right tool for the job!

Implementing Canonicals alongside robots meta instructions

Watch Out For:

Because you blocked crawling of that page via robots.txt

Canonicals that search engines can't "see"

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