Ratio of circumference to radius I know that the ratio of the circumference to the diameter is Pi - what about the ratio of the circumference to the radius? Does it have any practical purpose when we have Pi? Is it called something (other than 2 Pi)? 

The ratio of the circumference to the radius is $2\pi$, which some people call "One turn". I think you would enjoy to read this article: "$\pi$ is wrong!" by Bob Palais. Other people call $2\pi$ by the name of Tau. See this page: http://tauday.com
