A circuit with a few components quickly leads to a complicated expression for the transfer function. It is often sufficient, and of course, easier to work with approximations to the transfer function.
Let 
be the ratio of two polynomials
where
If one term dominates in each polynomial
Thus
We define 
and plot
which is a straight line on a log-log plot with integer slope.
As an example, consider our RCL circuit:
where 
and 
.
At low frequencies 
and
On a log-log plot
which has a slope of +1.
At high frequencies 
and
On a log-log plot
which has a slope of -1.