Having said all that, I should note that the connection between "hang out a shingle" and lawyers remains particularly strong. Thus, for example, from Edith Shillue, Peace Comes Dropping Slow: Conversations in Northern Ireland (2003):
His [law] firm was relatively new, and as he was really the "and Co." of of "R. P Crawford and Co.," I felt I should show him some support. Robbie's decision to "hang out a shingle" (which is our American slang for going it alone in the legal business) instead of staying with larger firms brought a smile to my face. My father was a lawyer who had hung out a shingle.
It may be that the persistence of this idiom in connection with the legal profession is tied to the fact that for many years (in the United States) lawyers were forbidden by law and/or by state bar rules from advertising their services. As a result, the shingle was one of the few acceptable ways of notifying the public of one's availability to provide legal services. This was especially a problem for lawyers setting out on their own practice, rather than joining an established firm.
