First, you must make clear that you are not Open Source Matters, Inc. or the Joomla! project and that you do not represent Open Source Matters, Inc. nor the Joomla! project. A simple disclaimer on your home page is an excellent way of doing that.
Second, you may not incorporate the Joomla! name or logo into the name or logo of your website, product, business or service.
Third, you may use the Joomla! name (but not the Joomla! logo) in descriptions of your website, product, business or service to provide accurate information to the public about yourself.
Fourth, you may not use the Joomla! logo.
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If you would like to use the Joomla! name or logo for any other use, please contact the Joomla! project and we'll discuss a way to make that happen. We don't have strong objections to people using the name for their websites and businesses, we just want to have a chance to review such use. Generally, we approve your use if you agree to a few things, mainly: (1) our rights to the Joomla! trademark are valid and superior to yours and (2) you'll take appropriate steps to make sure people don't confuse your website for ours. In other words, it's not a big deal, and short conversation (usually done via email) should clear everything up n short order.
If you currently have a website that is using the Joomla! name and you ave not gotten permission from us, don't panic. Let us know, we'll give you permission, as described above.