Do links between root domains and subdomains really pass PageRank?

Why is Google so vague on this issue?

The official statement carefully avoids resolving a fifteen-year-old debate: are links between root domains and subdomains treated as internal linking (full PageRank transmission) or external (with potential dilution)? This gray area is likely not a communication accident.

Google has every interest in maintaining this ambiguity. By refusing to formalize a strict rule, the search engine reserves the right to adapt its behavior to the context — type of site, thematic consistency, user signals. A root domain example.com pointing to blog.example.com will not necessarily be treated the same way as a link to shop.example.com or fr.example.com.

What does "contextual" really mean in this statement?

The term "contextual" suggests that Google evaluates these links on a case-by-case basis, based on qualitative signals rather than a binary technical rule. A subdomain hosting content thematically aligned with the main domain (e.g.: a corporate blog on blog.brand.com) will likely be treated differently than an isolated technical subdomain (CDN, third-party platform).

This contextual approach implies that the technical structure alone is not enough. Google likely analyzes editorial consistency, inter-domain navigation, user click signals, and even patterns of duplicate or cannibalized content. A subdomain that functions as a sealed silo risks being treated as a distinct entity, even if it shares the same root domain.

When does this ambiguity pose a real problem?

For multilingual or multi-regional e-commerce sites, this imprecision complicates the architecture strategy. Should one prefer fr.site.com or site.com/fr/? Both structures have their proponents, but no official data allows a clear conclusion regarding the pure linking aspect.

Media groups that use thematic subdomains (sport.media.com, tech.media.com) face the same dilemma. If Google treats these links as external, a significant portion of link juice dissipates. If the algorithm considers them as internal, the architecture allows for boosting certain strategic sections. Google's statement leaves this question hanging.

• Google neither confirms nor denies the internal or external status of links between root domains and subdomains
• The decision should rely on user experience and editorial consistency, not on linking assumptions
• The term "contextual" suggests a qualitative evaluation on a case-by-case basis, without a universal technical rule
• Field A/B tests remain the only reliable method to measure the real impact on a given site
• The ambiguity maintained by Google allows it to adapt its behavior according to the site context

Is this statement consistent with field observations?

Let's be honest: empirical tests show contradictory results. Some SEOs report PageRank transmission almost equivalent to classic internal linking between root domains and subdomains. Others observe significant dilution, similar to an external link with a nofollow attribute.

The problem is that these isolated tests never control all variables. The subdomain's theme, its crawl history, the presence of reciprocal links, direct traffic volume — all these factors likely influence algorithmic treatment. Google is not lying by saying it's contextual, but this answer is frustratingly unhelpful for strategic planning. [To be verified] remains the honest mention for any categorical claim on this topic.

What nuances should be added to this official position?

Google's recommendation — base the decision on UX and editorial consistency — is not false, but it dodges the question of pure SEO performance. However, a site migrating from subdomains to subdirectories (or vice versa) occasionally observes organic traffic variations of 15 to 30 %. These fluctuations are not random.

Google implicitly suggests that its algorithm is sophisticated enough to ignore technical structure and focus on qualitative signals. This is probably true for large brands with strong domain authority. But for an average site, technical structure still matters — and the maintained ambiguity leaves practitioners in uncertainty. If your subdomain is poorly crawled or perceived as a low-quality satellite, no editorial consistency will compensate.

In what cases does this rule absolutely not apply?

Technical subdomains (CDN, third-party SaaS platforms, staging environments) are clearly treated separately. A link from www.site.com to cdn.site.com or app.site.com has no reason to pass editorial PageRank — and no one claims otherwise.

Another edge case: subdomains used for spam or content farms. Google has repeatedly demonstrated its ability to algorithmically isolate a toxic subdomain without penalizing the root domain. In this scenario, the treatment is explicitly non-contextual but punitive. The ambiguity of the official statement clearly does not cover these extreme situations.

What should you do if you are hesitating between subdomains and subdirectories?

Start with a thematic consistency audit. If your potential subdomain shares the same editorial line, overall navigation, and branding as the main domain, a subdirectory is probably safer. The site.com/section/ structure eliminates any risk of algorithmic dilution.

If the content is really distinct — application platform, unmoderated UGC content, regional site with its own identity — the subdomain is justified for UX reasons. But don't expect a magical SEO boost through cross-links. Consider each subdomain as a semi-independent entity that will need to prove itself in terms of crawl, indexing, and ranking.

How can you verify that your current architecture is not penalizing your performance?

Analyze crawl data in Google Search Console: a subdomain that receives significantly less crawl budget than an equivalent subdirectory probably indicates differentiated treatment. Also compare organic performance (impressions, CTR, average position) between similar sections hosted on subdomains vs. subdirectories.

Scrutinize internal links via a crawler (Screaming Frog, Oncrawl): if your root domain hardly ever points to the subdomain, or vice versa, Google will struggle to detect editorial consistency. A weak inter-domain linking reinforces the separation signal, even if you wish otherwise.

What mistakes should you absolutely avoid in this context?

Don't multiply subdomains without a clear strategic reason. Each subdomain potentially dilutes the overall authority of the root domain and complicates crawl. A site with ten active subdomains but poorly linked together resembles more a network of satellite sites than a coherent architecture.

Also avoid duplicating content between root domain and subdomains without clear canonicalization. Google might treat these entities as competitors, risking cannibalization in the SERPs. If you use subdomains for language versions, rigorously implement hreflang tags — without that, Google won't understand the relationship.

These architectural optimizations can quickly become complex, especially on multi-level sites with a heavy technical history. In these configurations, support from a specialized SEO agency often helps avoid costly mistakes and finely calibrate the strategy according to your specific business objectives.

• Favor subdirectories (/section/) if editorial consistency and shared branding
• Only use subdomains if UX, technical, or regional distinctions are genuinely justified
• Measure crawl budget and the organic performance of each subdomain via Search Console
• Maintain a strong internal linking between root domain and subdomains to signal consistency
• Avoid content duplication across domains without appropriate canonicalization or hreflang
• Empirically test any migration between subdomain and subdirectory before global deployment