What makes a good slug
A slug is the human-readable part of a URL. It should be lowercase, use hyphens between words, contain only characters that survive a URL without escaping, and be short enough to read in a search result.
Hyphens rather than underscores: search engines have historically treated a hyphen as a word separator and an underscore as a joiner, somy_blog_post can be read as one long token wheremy-blog-post reads as three words.
Accents and non-Latin characters
The naive slug function strips anything outside a-z0-9, which turns "Café Münster" into "caf-mnster" — the accented letters vanish entirely. This tool normalises to decomposed Unicode first and removes only the combining marks, so the base letters survive: "cafe-munster".
Length and stop words
Google typically shows around 60–70 characters of a URL in search results, and the slug is only part of that. Removing stop words ("the", "and", "of") shortens a slug without losing much meaning, though it can make a title read oddly — it is a judgement call rather than a rule.
One thing worth being careful about: changing an existing slug changes the URL, which breaks inbound links and loses accumulated search standing unless you set up a redirect. Slugs are best decided once.