Assume 
and 
are two plane figures that are similar. Assume
that 
is a segment or arc of that corresponds to a segment
or arc 
of , and that 
is a segment or arc of that
corresponds to a segment or arc 
of . Then
In particular, the ratio of the circumference C to the diameter d
is exactly the same for every circle. We can then define 
to equal this ratio. That is to say, we define 
.
As a consequence, 
.