Nancy Gish wrote:

> I do not know the history of how "McDonalds" became a brand name, and
> brand names pay no attention as a rule to rules. But it could well be
> simply a plural rather than a possessive. If it simply is the name of a
> family, i.e., the McDonalds, then the possessive would be "Mcdonalds' "
> and would not be in the brand name.

The first McDonalds was owned by two brothers named (what else) McDonald.
A short history of the early chain, bought out by Ray Kroc, is at
    http://www.fiftiesweb.com/pop/mcdonalds.htm

At the webpage is a picture of the first restaurant and it displays a
sign saying (with no apostrophes):

 MCDONALDS
  FAMOUS
 HAMBURGERS

Regards,
    Rick Parker