What is a URL?
A URL is the address of a page or file on the web: it says how to reach it, which server holds it, and where on that server it is. It has up to five parts: the scheme, the host, the path, the query and the fragment. Two of them catch people out. The path is case-sensitive, so Google treats /Apple and /apple as different pages. And the fragment, the part after #, never reaches the server at all. I asked 4 sites I run for 28 variants of one page each, and every variant either redirected to the real address, returned 404, or pointed back to it with a canonical tag.
The five parts
| Part | What it does | Case matters? |
|---|---|---|
| Scheme (https) | Which protocol the browser uses | No |
| Host (shop.example.com) | Which server to ask. The port, if shown, says which door on it | No |
| Path (/guides/blue-shoes) | Which page or file on that server | Yes |
| Query (?color=navy) | Extra parameters for the server | Yes |
| Fragment (#sizing) | A spot within the page, handled by the browser only | Yes |
From RFC 3986, the standard that defines URL syntax, and MDN, read 9 November 2026.
The part after # never reaches the server
When you open a link with a fragment, the browser asks the server for the page without it, then scrolls to the matching spot itself. That's why Google says not to use fragments to change what a page shows: Google Search generally doesn't support them. A site that loads different content for #pricing and #features looks like one page to a search engine. Sites built as single-page apps should use real paths instead.
Different-looking URLs, same page
Google treats each distinct URL as a separate address, even when they show the same content. The usual variants:
- Case: /Guides and /guides are different URLs to Google. Pick lowercase and stick to it.
- Trailing slash: /guides and /guides/ are different, except at the very root of the site.
- Protocol: http:// and https:// versions are duplicates. Google prefers the https:// one.
- Host: www.example.com and example.com are variants of each other.
Google's advice is the same for all of them: choose one version, send the others there with a 301 redirect, and where a redirect isn't possible, point to the chosen version with a canonical tag.
What four real sites did with the variants
On 9 November 2026 I took one page on each of 4 sites I run and requested it 7 ways.
| Variant | What all 4 sites did |
|---|---|
| The address as published | Answered 200 |
| Without the trailing slash | Redirected with a 301 to the version with it |
| In capital letters | Returned 404 |
| With http:// | Redirected with a 301 to https:// |
| With www. | Redirected with a 301 to the address without it |
| With a tracking parameter added | Answered 200, with a canonical tag pointing to the clean address |
| With a doubled slash after the host | Answered 200, with a canonical tag pointing to the clean address |
That is 28 requests and no stray duplicate: every variant either ends at the real address or tells search engines which address is the real one. The last two rows are the ones to check on your own site. A server that answers 200 at a second address is fine only if the page names its canonical.
What makes a good URL
From Google's URL structure guidance:
- Use readable words, not long ID numbers.
- Separate words with hyphens, not underscores.
- Use as few parameters as you can. Long parameter strings make many URLs that show the same content.
- Keep one address per page, and redirect the rest.
URLs can be long. The HTTP standard recommends that software handle URLs of at least 8,000 bytes, and the sitemap protocol caps each sitemap address at under 2,048 characters. Short, readable addresses are easier to share and to read in search results.
What Check Engine checks
Check Engine reads every URL it crawls and reports URLs with a session ID in the query string, uppercase letters in the path, encoded spaces, more than 115 characters, or several query parameters. The URL structure guide explains each finding and which popular URL rules have no source behind them.
Sources
- RFC 3986, URI generic syntax
- MDN, what is a URL
- MDN, URI fragment
- WHATWG URL standard
- Google, URL structure best practices
- Google, to slash or not to slash
- Google, canonicalization
- RFC 9110, URI length
- sitemaps.org protocol
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Common questions
What is the difference between a URL and a URI?
A URI is any identifier for a resource. A URL is a URI that also says how to reach it, such as an https:// address. In everyday use, and in the web's own URL standard, the two mean the same thing.
Are URLs case-sensitive?
Partly. The scheme and the host are not: EXAMPLE.com and example.com are the same site. The path and the query are, so /Guides and /guides can be different pages, and Google treats them as different URLs.
Does the part after # in a URL reach the server?
No. The browser keeps the fragment and uses it to jump to a spot on the page. That's why Google says not to use fragments to change a page's content.
Should my URLs end with a slash?
Either is fine. What matters is choosing one form and redirecting the other to it, because Google treats /page and /page/ as different URLs everywhere except the site's root.