From Wikipedia's Robertson screwdriver entry:

Quote:
Today Robertson screws are almost unknown in the United Kingdom, scarce in the United States, where they are used mainly in theatrical set construction and account for 10% of screws sold, while being very common in Canada where 85% of the screws sold use the Robertson head.

FWIW, I've never heard them called "Robertson" before -- just "square drive" -- and they'd be more popular in the US, probably, if they didn't cost like 25% more than Phillips screws.
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Bitt Faulk