In a recent episode of Google’s Search Off the Record podcast, Allan Scott from the “Dups” team discussed how Google identifies the primary URL when duplicate pages are detected. He shared that Google uses approximately 40 different signals to choose the most appropriate URL from a group of similar pages.

About 40 Signals Used for Canonical URL Selection
Duplicate content is a common challenge for search engines, as many websites feature pages with similar or identical content. To address this, Google employs a method called canonicalization, which enables the search engine to designate one URL as the main version to index and display in search results.

Google has previously highlighted the significance of tools such as rel=”canonical” tags, sitemaps, and 301 redirects in the canonicalization process. However, the total number of signals used for this purpose is more extensive than many might assume.

Scott revealed during the podcast:
“While the exact number fluctuates, I estimate it’s around 40 signals.”

Some of the recognized signals include:

1. rel=”canonical” tags
2. 301 redirects
3. HTTPS vs. HTTP
4. Sitemaps
5. Internal linking
6. URL length

The impact and weight of each signal may differ, with certain signals, like rel=”canonical” tags, playing a role in both clustering and canonicalization.

Managing Conflicting Signals
Given the large number of signals at play, Allan acknowledged that it can be challenging for Google to select the canonical URL when signals contradict each other. He mentioned:

“When signals conflict, the system will lean on weaker signals.”

This means that while more authoritative signals like rel=”canonical” tags and 301 redirects are essential, other factors might influence the decision when these signals are ambiguous or inconsistent.

As such, Google’s canonicalization process requires balancing these signals to determine the correct canonical URL.

Best Practices for Canonicalization
To help Google identify the correct canonical URL, it’s important to follow best practices:

1. Properly implement rel=”canonical” tags.
2. Use 301 redirects for pages that have permanently moved.
3. Ensure the HTTPS version of pages is accessible and linked.
4. Submit sitemaps with the preferred canonical URLs.
5. Maintain consistency in internal linking.

These practices guide Google to find the appropriate canonical URLs, which aids in crawling, indexing, and improving search visibility for your site.

Common Mistakes to Avoid
Several common errors can cause issues with canonicalization:
1. Incorrect or conflicting canonical tags:

• Pointing to non-existent or 404 pages.
• Using multiple canonical tags with different URLs on a single page.
• Pointing to a completely different domain.

Fix: Double-check your canonical tags, use only one per page, and ensure absolute URLs are used.

2. Canonical chains or loops: When Page A points to Page B as its canonical, but Page B points back to Page A or another page, creating a loop.
Fix: Ensure that canonical tags always point to the final, preferred version of the page.

3. Using noindex with canonical tags: This sends mixed signals to search engines, as noindex indicates the page should not be indexed, making canonical tags irrelevant.
Fix: Use canonical tags for consolidation and noindex tags for exclusion.

4. Canonicalizing to redirected or noindex pages: Pointing canonical tags to pages that are redirected or set to noindex causes confusion for search engines.
Fix: Ensure canonical URLs point to indexable pages with a 200 status.

5. Disregarding case sensitivity: Variations in the casing of URLs can lead to duplicate content issues.
Fix: Maintain consistent casing across URLs and canonical tags.

6. Neglecting pagination and parameters: Paginated content and URLs with parameters can lead to duplication if not handled properly.
Fix: Use canonical tags that point to the first page or “View All” for paginated content, and ensure parameters are handled consistently.

Key Takeaways
While it’s unlikely that Google will release the full list of the 40+ signals used for canonical URL determination, the insights shared were valuable. Here are the main points:

• Google utilizes about 40 different signals to select canonical URLs, with rel=”canonical” tags and 301 redirects being among the most influential.
• When signals conflict, Google relies on secondary signals to make its decision.
• Clear and consistent use of canonicalization signals (tags, redirects, sitemaps, internal linking) is crucial for proper indexing.
• Common mistakes such as canonical chains, conflicting signals, or incorrect implementations can confuse search engines.

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