Hyphens vs Underscores in URL Slugs: What Google Says

Both characters are technically valid in a URL. Only one of them fits how search engines split a slug into words, and that difference has been documented by Google for well over a decade.

What Google says about the two characters

In a Google Search Central document on URL structure, the guidance is direct: use hyphens to separate words in a URL, and avoid underscores, because an underscore is treated as a word joiner.

The practical effect shows up in how a crawler reads the path. Given /content-writing-tips/, the words extracted are content, writing, and tips. Given /content_writing_tips/, the token is content_writing_tips, which matches almost nothing a person would search for. Google can still occasionally match underscored URLs, and it has improved at it over time, but you are relying on a fallback instead of using the format the engine asks for.

The same logic applies to other separators. Spaces encoded as %20 are ugly and create duplicate address variants. Plus signs are read as literal characters by some systems. Periods turn a slug into what looks like a file extension. Hyphens have no competing meaning in a path segment.

Why this matters for SEO

• Word boundaries give search engines clean tokens to index, which is how the words in the slug can support the page topic.
• Readable URLs earn more clicks on the results page and in forums, chat apps, and email, because people can see what they are about to open.
• Hyphenated slugs are easier to speak, dictate, and dictate back correctly when someone shares a link verbally.
• Consistency prevents duplicates. One page, one address, with no version split across underscore and hyphen variants.

None of this outweighs content quality. A slug is a small ranking signal at best, and it cannot rescue a thin page. It is simply a detail that costs nothing to get right the first time and is expensive to change afterwards, because changing a live URL means setting up a redirect.

Five practical rules for good slugs

1. Separate every word with a hyphen, and never use underscores.
2. Keep the slug short. Three to six meaningful words is usually enough, and stop words like "the" and "and" rarely earn their place.
3. Use only lowercase letters, digits, and hyphens. Strip accents and punctuation, and replace ampersands with "and".
4. Describe the page, not the campaign. "/email-marketing-checklist/" will still make sense in two years; "/email-marketing-checklist-v2-final/" will not.
5. Decide the slug once. If you have to change a published URL, redirect the old address to the new one with a permanent 301.

One more habit worth keeping: avoid letting your CMS generate dates or IDs in the path. A slug that includes the publishing date makes evergreen content look stale, and IDs give readers no information at all.

Turning a headline into a clean slug by hand is tedious, and it is easy to leave behind a stray apostrophe or double hyphen. The Slug Generator does the conversion instantly, handling accents, punctuation, and repeated separators, entirely in your browser.