Dashes or underscores in your URLs: What’s the difference for your SEO?

Why does Google make this distinction between dashes and underscores?

Google needs to segment URLs into usable tokens for semantic indexing. Dashes (-) function like spaces: they indicate a clear separation between two terms. Thus, the URL "running-shoes-men" will be divided into three distinct entities that Google can analyze individually.

Underscores (_) come from a programming convention where they are used to link elements of the same identifier. Google treats them as standard alphanumeric characters. "running_shoes_men" becomes a single block, as if you had written "runingshoesmen". The engine cannot extract individual semantic components.

What real impact does this have on the indexing of my content?

This difference directly affects Google’s ability to match your URLs with user queries. If someone searches for "running men", a page with the URL "running-shoes-men" is more likely to align with that intent, because Google identifies these terms separately in the URL.

With "running_shoes_men", Google only sees a single incomprehensible token. It can still index the page through its text content, title, or meta tags, but you lose the SEO signal that the URL itself might provide. This is a self-inflicted handicap.

Does this rule apply only to new sites?

Matt Cutts specifically mentions new sites because changing existing URLs leads to 301 redirects, potential loss of PageRank, and considerable technical risks. On a mature site with thousands of indexed URLs, often the game isn't worth the candle.

However, if you are launching a complete redesign or migrating to a new architecture, it's the perfect time to fix these details. Established sites can survive with underscores without major SEO disasters, but every new piece of content should adhere to the dash standard.

• Dashes are recognized as word separators, while underscores are seen as linking characters
• The URL contributes to the overall semantic signal of a page, even if its weight remains marginal
• Correcting existing underscores requires 301 redirects and involves technical risks
• Always prefer dashes for any new content or new URL structure
• This rule applies to file names, category slugs, and all URL segments

Is this statement consistent with real-world observations?

Absolutely. Empirical tests confirm that Google does not extract semantic components from URLs containing underscores. If you analyze competitive SERPs, URLs with dashes overwhelmingly dominate premium positions, particularly in niches where exact matching still matters.

That said, the impact of the URL in the overall algorithm remains marginal compared to content, backlinks, or domain authority. A site with underscores but exceptional content and strong links will always outperform a mediocre competitor with perfect URLs. Let’s keep things in proportion.

What nuances should be added to this recommendation?

Some CMS and frameworks impose technical constraints that make underscores unavoidable in certain contexts. For example, legacy e-commerce platforms generate product URLs with underscores by default. If your system cannot modify this logic without massive rewriting, focus on other more profitable SEO levers.

Moreover, domain names and subdomains do not accept multiple dashes or underscores according to standard RFCs. This rule only applies to the URL paths after the main domain. Do not confuse the two levels.

In what cases does this rule lose relevance?

On very short URLs or single-page sites, the impact becomes negligible. If your strategy relies on a strong brand domain and long content indexed based on its substance rather than its URL, you may be able to use underscores without measurable harm.

Similarly, certain technical sectors (software documentation, public APIs) use strict naming conventions where underscores are an integral part of resource identity. In these cases, technical consistency and readability for developers take precedence over marginal SEO optimization.

What should I do concretely to optimize my URLs?

If you are creating a new site, set up your CMS from the start to generate slugs with dashes. WordPress, Shopify, and most modern platforms do this natively, but check permalinks settings. Some themes or plugins may impose alternative formats.

For existing sites, audit your URLs using Screaming Frog or your preferred crawler. Identify strategic pages with underscores that generate significant organic traffic. Prioritize migrating commercially high-potential content, not blindly striving for completeness.

What mistakes should be avoided during a URL migration?

Never redirect with 302 or 307, only with permanent 301. Google needs to understand that the old URL is definitively replaced; otherwise, PageRank and historical signals will not transfer correctly. Test each redirect individually before global deployment.

Avoid redirect chains (A→B→C). If a URL with an underscore was already pointing to another page, create a direct redirect to the final destination with dashes. Each additional link dilutes the signal and slows the crawl.

How can I check that my site adheres to this standard?

Export all your URLs from Google Search Console (Performance > Pages). Filter those containing the "_" character with Excel or a Python script. Cross-reference this list with your traffic data to identify high-stakes pages.

Additionally, set up an alert in your monitoring to detect any newly created URLs with underscores. This can happen due to a misconfigured plugin, an import of external data, or human error during content creation.

• Set the CMS to generate slugs with dashes by default
• Audit existing URLs and prioritize pages with high traffic or conversion
• Implement 301 redirects tested individually before deployment
• Check for the absence of redirect chains (maximum 1 hop)
• Monitor Search Console for any 404 errors post-migration
• Submit the new XML sitemap to speed up recrawling