I have always, so far, posted without thinking about tagging.

Is there any information on how the Tagging system works and how I should use it?

Also, is there an easy way of listing all the existing tags, apart from just looking at the "Popular Tags" item in the sidebar on the main page?

Finally, can I now go back to an existing post or topic and add tags now?

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Here's some information on tagging back when IPS started to introduce it to the public.

To be honest, I'm still learning about tagging as well. I may be able to add pre-selectable tags but so far I do most of it manually. If you're able to edit posts (even years back) you should be able to add tags to it.

OK, thanks, I'll try it out!