Do underscores in URLs really harm SEO performance?

What distinction does Google make between underscores and hyphens?

Google treats hyphens as word separators, enabling the search engine to distinguish each term in a URL. For instance, "seo-technique" will be analyzed as two distinct words: "seo" and "technique". Underscores are considered connectors, merging the terms into a single word: "seo_technique" becomes "seotechnique".

This distinction has existed since the beginning of the search engine. At that time, underscores were viewed as a programming character rather than a natural separator. Hyphens better mimic the behavior of spaces in written language, which explains their favored treatment.

Why hasn’t Google corrected this technical limitation?

Matt Cutts' statement reveals that Google evaluated a technical evolution to treat underscores as hyphens. The project was abandoned due to lack of strategic priority. In practice, development resources have been allocated to projects deemed more impactful for result quality.

This decision reflects a simple reality: the proportion of sites that heavily use underscores in their URLs remains marginal. The effort to develop, test, and deploy such a change did not justify the expected benefit. Google focuses its investments on high-impact algorithms, such as semantic understanding or quality content detection.

Is the impact really negligible for all sites?

Matt Cutts emphasizes that the impact is "relatively low", a phrase that deserves attention. Low does not mean none. In ultra-competitive queries where every micro-signal counts, a site structured with underscores could theoretically lose a few positions to an otherwise identical competitor using hyphens.

In practice, this difference remains masked by hundreds of other more determining ranking factors. Content quality, domain authority, user behavior, and semantic structure largely overshadow this URL detail. A site with underscores but excellent content will always outperform a mediocre site with perfect URLs.

• Hyphens separate words: each term in the URL is analyzed individually by Google
• Underscores merge words: Google reads them as a single term with no spacing
• The impact remains marginal: no active penalty, just a slight loss of semantic granularity
• No migration recommended: for an existing site, the risks of URL restructuring outweigh potential gains
• For a new project: always prioritize hyphens by default

Does this statement align with real-world observations?

Empirical tests largely confirm this stance. Comparative studies on identical sites with underscores versus hyphens show statistically insignificant ranking differences. Sites using underscores continue to rank normally for their target keywords, even in competitive queries.

The nuance lies in the long tail. For specific queries where URL terms serve as semantic anchors, a hyphen may theoretically provide a slight advantage. However, this gain remains imperceptible against the natural daily fluctuations of SERPs. [To verify]: no public data quantifies this gap precisely, which is more theoretical than measurable observation.

When should one still consider changing?

The migration from underscores to hyphens is only justified in three specific scenarios. First case: a comprehensive technical overhaul is already planned, in which case integrating this change incurs only a marginal cost. Second case: the site generates dynamic URLs that are technically easy to modify without impacting the indexed history.

The third case, rarer: an e-commerce site with thousands of automatically generated URLs including underscores, where a correction of the URL template can be gradually deployed without massive redirection. In all other cases, the benefit/risk ratio clearly tilts against migration.

What is Google's actual position on URLs in general?

Google has been reiterating for years that readable and descriptive URLs help marginally in ranking, but especially in click-through rates. A clear URL reassures users in the SERPs and increases CTR, which indirectly influences positioning through behavioral signals.

The underscore versus hyphen debate often obscures the main point: the URL structure should primarily serve the user. A short, comprehensible URL containing main keywords far surpasses any separator detail. Google values coherence and architectural simplicity long before the syntactical perfection of each URL segment.

What should I do if my site uses underscores?

Don't change anything. Migration presents technical and SEO risks greater than potential gains. Each 301 redirect slightly dilutes PageRank, and a poorly configured chain of redirects can fragment your link juice. Mapping errors between old and new URLs frequently result in brutal traffic losses.

Focus your resources on optimizations with measurable impact: improving content, acquiring quality backlinks, optimizing loading speed, enhancing user experience. These projects yield traffic and conversion gains far superior to a hypothetical optimization of underscores.

How to structure URLs for a new project?

For any new site or new section, always use hyphens as separators. This convention has become the web standard, recognized by both engines and users. URLs with hyphens are more readable, easier to communicate orally, and avoid any technical ambiguity.

Keep URLs short and descriptive: 3 to 5 words maximum, containing only essential terms. Avoid stop-words (the, a, of, to), special characters, and technical identifiers. An optimal URL looks like "domain.com/category/main-keyword", where each segment provides a clear semantic meaning.

What errors to avoid during a potential migration?

If you still decide to migrate, a comprehensive URL mapping is the first critical step. Each old URL must be mapped to its new version, without exception. Crawling tools (Screaming Frog, Sitebulb) allow for extracting all indexed URLs and generating a correspondence matrix.

Test redirects in a staging environment before any production deployment. Ensure each redirect returns a HTTP 301 code (permanent), not a 302 (temporary). Monitor the Search Console daily for three weeks following the migration to immediately catch any 404 or soft 404 errors.

• Audit your current URLs: identify those using underscores and assess their performance
• Do not migrate without a strategic reason: the underscore alone never justifies a complete architecture overhaul
• For a new site: configure hyphens as separators in your CMS from the start
• If migration is necessary: create a complete old/new matrix and test each redirect
• Monitor the Search Console: track 404 errors and indexing losses post-migration
• Prioritize consistency: it’s better to have underscores everywhere than mixed conventions that cause confusion