Given that this is such a freewheeling and decentralized project, there's nothing to keep non-MLS' from participating. Many libraries have non-MLS staff on the reference desk and they usually do a very good job. My main request is that, if you participate, make sure you note the authority of your sources (like you should when you work the Reference Desk) and play up the fact that the asker's local library is also a good place to ask questions like the one you just answered. The entries on the "signatures & tag lines" page of this wiki mostly play up librarians, but they could play up libraries in general just as well, without getting into the whole MLS issue.

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