Google's faceted navigation guide prefers prevention: disallow filter URLs in robots.txt and keep crawlable only item pages plus one unfiltered listing page, or use URL fragments, which Google generally does not crawl. rel=canonical and nofollow are weaker, slower options.
Thing · Directive
rel=canonical
A link element that names the preferred URL among duplicates; one of several signals Google weighs when it picks the canonical.
- Claims
- 41
- In Google’s docs
- 7
- Said at the event
- 26
- Not in docs
- 5
- Kit items
- 13
Glossary · rel=canonical
A link element in the head of a page that names the site owner's preferred URL. Google treats it as a strong signal, not an order.
Google’s documentation 7
Documented in
- How to specify a canonical URL with rel="canonical" and other methods
- 5 common mistakes with rel=canonical
- Specify your canonical
Google's canonical guide ranks the ways to signal a preferred canonical by strength: redirects and rel=canonical annotations are strong signals, sitemap inclusion is a weak signal, and combining methods makes them more effective.
Google Search Central · Day 2 · Handling web duplication
Google's 2013 post on rel=canonical mistakes says pointing rel=canonical from page 2 or later of a paginated series to page 1 is incorrect because the pages are not duplicates, and that it would result in the content on later pages not being indexed at all.
Search Central blog (8 April 2013) · Day 2 · Handling web duplication
Google's guide to specifying canonical URLs says Google supports explicit rel=canonical link annotations as described in RFC 6596, the canonical link relation specification.
Google Search Central · Day 2 · Lightning session E: Managing Duplicates and Site Moves
Google's 2009 Search Central blog post introducing rel=canonical says a canonical URL may be a URL that redirects: Google then processes the redirect as usual.
Search Central blog (12 February 2009) · Day 2 · Lightning session E: Managing Duplicates and Site Moves
Google's 2013 Search Central blog post on rel=canonical mistakes says to specify no more than one rel=canonical per page: when a page has more than one, Google will likely ignore all of them, and any benefit of a legitimate canonical is lost.
Search Central blog (8 April 2013) · Day 2 · Lightning session E: Managing Duplicates and Site Moves
Google's 2009 Search Central blog post introducing rel=canonical says Google's algorithm is lenient and can follow canonical chains, but strongly recommends updating links to point to a single canonical page for optimal canonicalization.
Search Central blog (12 February 2009) · Day 2 · Lightning session E: Managing Duplicates and Site Moves
Said at the event 26
Slide and stage claims that name it, the ones Google’s documentation does not cover first.
Label
Grade
Not in docs 5
Google's speaker said that pointing rel=canonical from the pages of a paginated set to the first page can sometimes make sense depending on the goal, for example to make a category page more visible, but that it affects canonicalization and deduplication.
Google's speaker suggested checking rel=canonical links with a crawler such as Screaming Frog to make sure they are reasonable.
Canonical chains can run through canonical links alone, through a server-side redirect between members, or through a client-side redirect between members.
Tobias Schwarz · Day 2 · Lightning session E: Managing Duplicates and Site Moves
Canonical loops, such as two HTML pages whose canonical tags point to each other, are a structural conflict: the group has no canonical leader, so its canonical information cannot be used.
Tobias Schwarz · Day 2 · Lightning session E: Managing Duplicates and Site Moves
Canonical loops can also run through server-side or client-side redirects, not only through canonical tags.
Tobias Schwarz · Day 2 · Lightning session E: Managing Duplicates and Site Moves
Consistent with docs 11
Google builds duplicate clusters from four kinds of input: redirects, content, rel=canonical, and a 'magic bucket' of other things.
rel=canonical is probably the most common way site owners signal duplicates, but it is very often wrong, for example a tag whose value reads 'canonical target' instead of a real URL (the example is partly unclear in the recording), so Google can only sometimes trust it.
Consistent with docsD2-C387
Google named three considerations when picking a representative URL: hijacking across pages or sites, user experience (the page can load, meta refresh, security) and site-owner signals (redirects, rel=canonical, sitemaps).
Consistent with docsD2-C390
Clear site-owner signals about which URL should be canonical make a big difference to Google's choice; the speaker named redirects, listing only the preferred URL in sitemaps, and rel=canonical, which the speaker said also helps a bit.
Consistent with docsD2-C396
Google's closing suggestions on duplication: use redirects for site migrations, use HTTP result codes, don't block agents, check your rel=canonical links, use hreflang links to help Google localize, report weird canonicals in the forums, make secure pages that work, and keep canonical signals clear.
According to the canonical link specification cited by a community speaker, when a canonical tag is declared improperly, the application that processes it may apply its own heuristic and decide what to do instead.
Tobias Schwarz · Day 2 · Lightning session E: Managing Duplicates and Site Moves
Consistent with docsD2-C415
The crawler report shown on screen flagged a URL discovered only through a canonical tag, with no internal a href links leading to it, as a phantom document outside the visible site structure.
Tobias Schwarz · Day 2 · Lightning session E: Managing Duplicates and Site Moves
Canonical chains, where a page's canonical target leads on to yet another URL, are named as improper use in the canonical link specification, according to a community speaker.
Tobias Schwarz · Day 2 · Lightning session E: Managing Duplicates and Site Moves
Consistent with docsD2-C422
The crawler report shown on screen flagged a canonical chain as a problem: canonical links that form a chain instead of all pointing directly to the group's canonical leader.
Tobias Schwarz · Day 2 · Lightning session E: Managing Duplicates and Site Moves
When internal links point only to page A and the canonical leader is reached only through A's canonical link, the leader is reachable by machines but not by human visitors, a signal conflict that asks the search engine to index a page users cannot reach.
Tobias Schwarz · Day 2 · Lightning session E: Managing Duplicates and Site Moves
Canonical links between different language versions of a page are a misuse: if a search engine accepted such a canonical group, only one language version would rank.
Tobias Schwarz · Day 2 · Lightning session E: Managing Duplicates and Site Moves
Confirmed by docs 8
Confirmed by docsD2-C031
Google extracts the rel=canonical link, through which site owners state their preferred canonical URL, and uses it in deduplication and in canonical selection.
Confirmed by docsD2-C032
The rel=canonical link element is placed in the head section of the HTML and tells Google that one page is the representative of another.
Many SEOs assume Google must follow their rel=canonical, but because people sometimes get it wrong, Google has to make its own judgment about the canonical.
Broken canonical tags can make the wrong pages of a site show up in search results.
Tobias Schwarz · Day 2 · Lightning session E: Managing Duplicates and Site Moves
Language versions of a page should be connected with hreflang annotations instead of canonical links.
Tobias Schwarz · Day 2 · Lightning session E: Managing Duplicates and Site Moves
A community speaker advised fixing canonical chains by cleaning up the canonical graph so that every canonical link points directly to the canonical leader.
Tobias Schwarz · Day 2 · Lightning session E: Managing Duplicates and Site Moves
A community speaker said that, under the canonical link specification, an improperly declared canonical tag can also be ignored completely by the application that processes it, not only replaced by that application's own heuristic.
Tobias Schwarz · Day 2 · Lightning session E: Managing Duplicates and Site Moves
Not-indexed reasons in the Page indexing report include pages excluded by a noindex rule and 'Alternate page with proper canonical tag'.
Nothing to verify 2
StageD2-C407
In a community speaker's terms, a canonical group is the set of all URLs connected through canonical links, and its canonical leader is the page that all the group's canonical links ultimately point to.
Tobias Schwarz · Day 2 · Lightning session E: Managing Duplicates and Site Moves
The crawler report shown on screen rated a canonical group member that issues a redirect instead of displaying content as an error, and flagged canonical links whose target returns a non-200 status or is itself a redirect.
Tobias Schwarz · Day 2 · Lightning session E: Managing Duplicates and Site Moves
Press and analysis 8
AnalysisD2-C035
Put rel=canonical and hreflang link elements in the head of the HTML the server sends, not only in JavaScript-rendered HTML, so Google can read them during HTML parsing without depending on rendering.
Ibrahim Anjro · Day 2 · How is HTML interpreted
AnalysisD2-C392
Google's duplication talk described rel=canonical as something that 'also helps a bit', while Google's canonical guide calls it a strong signal alongside redirects and calls sitemap inclusion weak; treat redirects and rel=canonical as the main levers and sitemaps as support.
Ibrahim Anjro · Day 2 · Handling web duplication
AnalysisD2-C400
Before a migration or template change, align every canonical signal for the preferred URL (redirects, rel=canonical, internal links, sitemap entries, hreflang and working HTTPS); Google says it follows the site owner only when the signals agree.
Ibrahim Anjro · Day 2 · Handling web duplication
AnalysisD2-C404
A broken canonical setup does not fail visibly: pages still load normally while the choice of URL passes to the search engine's own heuristics, so canonical tags need a regular audit with a crawler and the URL Inspection tool.
Ibrahim Anjro · Day 2 · Lightning session E: Managing Duplicates and Site Moves
AnalysisD2-C416
To audit canonicals at scale, resolve every crawled URL to the final URL its canonical links and redirects lead to, group URLs by that leader, and flag groups with more than one hop, a loop, several canonicals on one page, or a leader that is noindexed, blocked, non-200 or redirecting.
Ibrahim Anjro · Day 2 · Lightning session E: Managing Duplicates and Site Moves
AnalysisD2-C423
After a migration or a subdomain change, update rel=canonical tags to the new final URLs: canonicals still pointing at old, redirecting URLs create chains through server-side redirects like the one in the real-world example shown.
Ibrahim Anjro · Day 2 · Lightning session E: Managing Duplicates and Site Moves
AnalysisD2-C439
The crawler report shown at the talk rated a redirecting canonical target as an error, while Google's 2009 guidance accepts a canonical URL that redirects and says Google can follow canonical chains; the gap is one of severity, since both recommend pointing every canonical link straight at the final URL.
Ibrahim Anjro · Day 2 · Lightning session E: Managing Duplicates and Site Moves
AnalysisD2-C440
Google says canonicalization consolidates link signals from duplicates into the chosen canonical, so a leader with few links of its own is not necessarily weaker once Google accepts it; the practical risk of a weakly linked leader is that Google, treating rel=canonical as a hint, picks a different page as canonical.
Ibrahim Anjro · Day 2 · Lightning session E: Managing Duplicates and Site Moves
Built on these claims 13
Kit items about rel=canonical: their own words name it, or several of the claims they rest on do.
Developer requirements 10
Point every canonical directly at a final URL that returns 200, is indexable and is linked
Rests on 8 claims, 7 of them naming rel=canonical
Put exactly one absolute rel=canonical in the server-rendered head of every indexable page
Rests on 8 claims, 6 of them naming rel=canonical; its own words name rel=canonical
Audit canonical groups with a crawler on a schedule
Rests on 5 claims, 4 of them naming rel=canonical; its own words name rel=canonical
Make all canonical signals agree: redirects, rel=canonical, internal links, sitemaps and hreflang
Rests on 6 claims, 3 of them naming rel=canonical; its own words name rel=canonical
Serve one host over HTTPS with a valid certificate and redirect every other origin to it
Rests on 4 claims, 2 of them naming rel=canonical
5 more
Do not canonicalise one language version to another; connect them with hreflang
Rests on 4 claims, 2 of them naming rel=canonical
Publish XML sitemaps listing only canonical, indexable URLs with accurate lastmod
Rests on 11 claims, 2 of them naming rel=canonical
Give each page of a paginated series its own URL, its own canonical and an <a href> to the next page
Rests on 6 claims, 2 of them naming rel=canonical
Never leave staging, preview or mirror copies of the site publicly crawlable
Rests on 2 claims, 1 of them naming rel=canonical
Keep faceted navigation out of the crawl with robots.txt rules or URL fragments
Rests on 12 claims, 1 of them naming rel=canonical; its own words name rel=canonical
Also inglossary terms Canonical chain and canonical loop, rel=canonical, Faceted navigation
Connected things 14
Relations
- Affects Canonicalization
2 claims, 2 documented
- Confirmed by docsD2-C031
Google extracts the rel=canonical link, through which site owners state their preferred canonical URL, and uses it in deduplication and in canonical selection.
Google's canonical guide ranks the ways to signal a preferred canonical by strength: redirects and rel=canonical annotations are strong signals, sitemap inclusion is a weak signal, and combining methods makes them more effective.
Most often named with it
Things named in the same claim, with the number of claims they share.
- Canonicalization 26
- Redirects 14
- Sitemaps 5
- hreflang 4
- Duplicate content 3
- Disallow 1
- Faceted navigation 1
- nofollow 1
- noindex 1
- Page indexing report 1
- Rendering 1
- robots.txt 1
- URL Inspection 1
Topics that feature it
- Canonical graphs: chains, loops and leaders 25
- Canonical selection 22
- Redirects, site moves and alternate names 9
- Duplicate content 4
- Sitemaps 3
- hreflang annotations 3
- Pagination and "load more" 2
- Processing: from fetched page to index 2
- URL discovery 2
- Faceted navigation 1
- Indexing statuses in Search Console 1
- Page language and language versions 1