Tau is an alternative to π, usually represented by the Greek letter tau (τ), is the ratio of a circle's circumference to its radius (instead of its diameter). This constant is the number of radians in a circle, so the central angle of a fraction of a full circle is that same fraction of τ radians. Tau's proponents argue this direct relation makes learning about angles expressed in radians easier than with π, where the fraction must be doubled. Though it has conventionally been written as the product "2π", τ appears in many commonly used formulas.

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