When using a slide rule, keeping track of the number of digits to the left of the decimal point (DLDP) in the result is fairly simple if you know the basic rule

When using a slide rule, keeping track of the number of digits to the left of the decimal point (DLDP) in the result is fairly simple if you know the basic rule: For multiplication, the DLDP in the result is: - the sum of the DLDPs of the multiplicands MINUS 1 if the multiplication is done with the slide sticking out to the right of the ruler's body (for example 2.0 x 3.0 = 6.0). - the sum of the DLDPs of the multiplicands if the multiplication is done with the slide sticking out to the left of the ruler's body (for example 5.0 x 4.0 = 20.0). There's a similar rule for division, but that's left as an exercise for the student.