A logarithm is the inverse of an exponential function. The logarithm of a positive number m is written as logbm and it is read as the logarithm of m with base b.
A common logarithm is a logarithm of base 10. For example, log101000 is called the common logarithm of 1000.
It is equal to 3 because 103 is 1000.
Since common logarithms are used so often, the base does not need to be written.
In the above identity, m is a positive number. Recalling the definition of a logarithm, the common logarithm of m can be defined for positive values of m.