Just wondering: why is the forum id part of the url to open an existing topic?
I mean like when - on the main page with the topic listing - you click on a link to open an existing topic "&forum=6" is part of the url. But even when you change 6 into 7 or another number, the topic will still open, because the topic has its own id I suppose. The forum id seems to me like an unnecessary attribute in the url.

For me it's hard to remember why it has been coded that way initially and working since ancient times :) I see now there are perhaps 2 points on keeping the forum ID in the URL:

1) It allows to make well-structured forums hierarchy specially when it's generated using .htaccess (like here on our forums, you may notice that each topic is held like under its own sub-folder of the forum) - more related keywords in the URL could make the topic's URL more appealing to a search engine.

2) When topic is moved to another forum, it's URL changes and it's a signal to the search engines that something has been changed to the topic (when it's moved to another forum, this also affects the indexed content of a forum in whole). If there would be just a topic ID supplied in the URL, there would be not possible to catch the point if a topic has been moved or not.

I understand that the topic also could run without forum ID as its effect on even the 2 points above is not that powerful. But I doubt that I'd change this approach now. It affects nothing in the same manner as it would affect something important.

Ok, that makes it clear to me. I now understand why this is. Thank you.