Why does the URL to an answer include the question ID?

Question

On Stack Overflow the link to an answer looks like this

https://stackoverflow.com/a/<answer-id>/<question-id>
// https://stackoverflow.com/a/69119825/819887

Out of curiosity:

• Why was this format chosen (instead of just https://stackoverflow.com/a/<answer-id>/)
• Are there any speed gains to include both ids (<question-id> and <answer-id>)?
• Is there an article that explains why this URL scheme was used?

Answer 1

It doesn't. 819887 is a user ID, to be precise the one who clicked on the 'Share' link underneath the answer. You can see it in your profile as well: https://stackoverflow.com/users/819887/surfmuggle. This is to keep track of referrals for the Announcer badge and family.

Now, when you visit that link, you'll see that the final URL does contain both the question and answer ID:

https://stackoverflow.com/questions/69119268/how-do-i-roll-to-a-specific-browser-version-with-playwright/69119825#69119825

69119268 is the question ID, that's the canonical URL for SEO purposes. The answer ID 69119825 appears twice: once to make sure that you're on the right page if there are lots of answers; the #69119825 is the slug which makes your browser scroll to that answer.

You might be interested in Documentation for Stack Exchange engine URLs? and List of unlinked pages on Stack Exchange sites to learn more.