Collier's New Encyclopedia (1921)/Arithmetical Complement

ARITHMETICAL COMPLEMENT, that which a number wants to make it reach the next highest decimal denomination. Thus the arithmetical complement of 4 is 6, for 4 + 6 are = 10, and that of 642 is 358, because 642 + 358 are = 1,000. The arithmetical complement of a logarithm is what it wants to make it reach 10.