In general, when we write a number N in the scientific notation of the form a × 10ⁿ the “a” part is a number greater or equal to 1 but less than 10 and hence its logarithm can be read from a common logarithm table. This logarithm is a positive decimal and is called the mantissa of the logarithm of the number. The factor, 10ⁿ is an integral power of 10 and hence its logarithm is its exponent, n. This integral value of logarithm of the number is called the characteristic of the logarithm. Therefore, the common logarithm of a number is equal to the characteristic plus the mantissa.

The common logarithm table is a table of mantissas. The mantissa can be given to any number of decimal place, but in most of the logarithm tables it is often given only to four decimal places.