Many slide rules have the sign (Prod.)/(−1) at the right-hand end of the
D scale, while on the left is (Quot.)/(+1.) It is somewhat unfortunate
that these signs refer to rules for determining the number of digits in
products and quotients, which are used to a considerable extent on the
Continent, and conflict with those used in this country. By the
Continental method the number of digits in a product is equal to the sum
of the digits in the two factors, if the result is obtained on the LEFT
_of the first factor_; but if the result is found on the RIGHT of the
first factor, it is equal to this sum − 1. The sign (Prod.)/(−1) the
_right_-hand end of the D scale provides a visible reminder of this
rule.
Similarly for division:—The number of digits in a quotient is equal to
the number of the digits in the dividend, minus those in the divisor, if
the quotient appears on the RIGHT _of the dividend_, and to this
difference + 1, if the quotient appears on the LEFT of the dividend. The
sign (Quot.)/(+1) at the _left_-hand end of the D scale provides a
visible reminder of this rule.