```Issue #4897 has been updated by phluid61 (Matthew Kerwin).

> Why is it called as ??, half of ???

It's actually two of ??.  The reason for the name is justified here: http://tauday.com/tau-manifesto#sec:one_turn

In summary: tau is the first letter of the Greek word "tornos" (lathe), which is the root of the English word "turn;" and the tau constant (2*PI) is the ratio of a circle's radius to its circumference (i.e. one _turn_.)  Also "the horizontal line in each letter suggests that we interpret the ???legs??? as _denominators_, so that ?? has two legs in its denominator, while ?? has only one. Seen this way, the relationship ??=2?? is perfectly natural."
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Feature #4897: Define Math::TAU and BigMath.TAU. The "true" circle constant, Tau=2*Pi. See http://tauday.com/
https://bugs.ruby-lang.org/issues/4897#change-37094

Author: sbaird (Simon Baird)
Status: Feedback
Priority: Low
Assignee:
Category: Joke
Target version: Next Major

Firstly please read the Tau Manifesto at http://tauday.com/ . It's quite long but essential to understanding why this is a good idea.

Here is a patch on trunk that implements this:
http://simonbaird.blogspot.com/2011/06/tau-in-ruby.html

Allow me to anticipate and respond in advance to some common objections:

1. It's easy to define it yourself so why put this in core.
Possibly correct, but I think this is the right thing to do. Tau is important. And it's a pretty small patch.

2. If this constant goes in then pretty soon someone will want every other math constant and there are hundreds of them. (Slippery slope argument).
The circle constant is one of the two most important numbers is mathematics. It's not just another math constant. We already define Pi.

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http://bugs.ruby-lang.org/

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