Default editor - how is it set?

For some reason my copy of tin has started selecting nano as my editor
instead of vi.  I have never had either EDITOR or VISUAL set in my
environment so I'm at a loss to understand why it has changed.

How does tin decide what editor to use?

The only thing I can think of that has changed recently which might
affect this is that I have moved vi from /usr/local/bin to /usr/bin.

Any suggestions welcome.

-- 
Chris Green