The behaviour for document extensions usually can be configured by the server administrator and I would guess a number have things set up to work with both as well as perhaps having a few default documents, eg. index.html, index.htm, default.asp, etc. the server can look for when a user for example requests http://www.mudcat.org

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Re case, I did cause myself some confusion with that when I moved from asp to php. ?TopicID and ?topicid became different things...