 According to Huntington (1933: 278), the term “Boolean algebra” was introduced by Henry M. Sheffer (1882–1964) in the paper where he showed that one could give a five-equation axiomatization of Boolean algebra using the single fundamental operation of joint exclusion, now known as the Sheffer stroke (see Sheffer 1913). Whitehead and Russell claimed in the preface to the second edition of Principia that the Sheffer stroke was the greatest advance in logic since the publication of Principia. (Hilbert and Ackermann (1928), by contrast, stated that the Sheffer stroke was just a curiosity.) Neither realized that decades earlier Schröder had discovered that the dual of the Sheffer stroke was also such an operation—Schröder’s symbol for his operation was that of a double-edged sword.