On 04 Apr 2007 12:51:52 +0100 (BST), David Richerby
wrote:

Not to be picky but I suspect that whether `they' replaced perpetual
check between 1954 and 1970 depends on who `they' a the USCF and
FIDE often have significantly different opinions about details such as
these.

Thanks. The 1970 edition of the book by Harkness mentions that
perpetual check is not a rule, in a kind of vague way that makes me
think that he had more to say about it in an earlier edition.
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