Commonly used logarithm is also called "decimal logarithm". Logarithms based on 10 are represented by the symbol "lg". For example, lgA represents the logarithm of a with the base of 10, where a is a real number. The common logarithm of any positive number can be expressed as the sum of an integer and a positive pure decimal (or zero); The integer part is called the "first number" of logarithm, and the positive pure decimal (or zero) is called the "mantissa" of logarithm. There is a logarithmic table of common logarithms.

Positive numbers are expressed by scientific notation as the product of the decimal of a one-digit integer and the integer power of 10, and then the common logarithm is taken.

For example, LG 200 = LG (102 * 2) = LG102+lg2 = 2+0.3010.

lg20=lg( 10^ 1*2)=lg 10^ 1+lg2= 1+0.30 10

lg0,002=lg( 10^(-3)*2)=lg 10^(-3)+lg2=-3+0.30 10