the forum with duplicate or off-topic posts. Many users do not bother to search a forum and directly create new threads to seek an answer to a question. On some forums, when the user types a new thread subject, the forum software brings up similar threads automatically on the side. This helps keep the number of redundant threads (or the overall forum pollution) to a lower level as users who neglect to search for a topic and are posting a thread may find the answer to their question as they are creating the new thread. Forum spam defenses Most forums are at risk of continuous attack by forum spammers, largely promoting websites with no relevance to the forum's niche. Systems vary in how they are geared to defense, and checking the offering is an important consideration before selection. A forum cannot succeed unless there is an effective system of defense, and an efficient set of tools for spam removal. CAPTCHAs are a common feature used among most internet forum software and are often used to prevent automated registrations. Banning or deleting membership should come as standard, with the ability to blacklist the username, email address or IP address for variable time spans. Reference to an anti-forum spam database can be built into the forum software, or offered as add-ons for specific databases. User-friendly URLs Human-friendly forum URLs do not have a query string and instead contain only the path of the topic. A user-unfriendly URL may contain cryptic parameters, numeric IDs, or file type extensions (e.g. .php) that do not matter to the user and could change if the forum is reimplemented using a different programming language. User-friendly URLs are easy to remember and to type, and may enhance search engine optimization (SEO). user-unfriendly URL example: http://example.com/forum/index.php?t=rview&th=120029 user-friendly URL example: http://example.com/usability-issues/user-friendly-urls In general this is accomplished via URL mapping, however, historically in many forum software packages, human-friendly URLs are an afterthought implemented via URL rewriting, and URLs often contain a numeric ID which represents the thread, while the remainder of the URL can in reality be any string: http://example.com/forum/12345/lets-use-friendly-urls and http://example.com/forum/12345/bogus-path-actually point to the same thread, http://example.com/forum/12345/. These are also known as URL slugs. The only non-controversial counter-argument to using user-friendly URLs is that they would be leaked in the HTTP referer header field when a user clicks on an external link from a post, which is undesirable for private