Radian

Radian, radial major second, or a radial whole tone is an interval of [math]\displaystyle{ \frac{1200}{2\pi} }[/math], or 190.98593 cents.

The interval has an interpretation that relates to all EDOs. Since pitch classes in all equal divisions of the octave form in the shape of a circle, radian interval therefore occurs as the radius of this perceptional circle.

Radian is 13 cents below just 9/8 and 9 cents below 12edo major second of exactly 1/6 of a circle, or 200 cents.

Approximations

Closest equal temperament approximations of the radian can be derived from the continued fraction of 1/2pi: 4\25, 7\44, 53\333, and 113\710. 7\44 and 113\710 are complementary to the historically notable 22/7 and 355/113 approximations of pi.

Other approximations include 5\31, which is also for the meantone, 11\69, the local meantone as well, and 13\82. Starting with 88edo, difference between the radian (14 steps out of 88) and 9/8 (15 steps out of 88) is visible.

Relative Radian

The Relative Radian is an interval based on a given interval. In being the interval of [math]\displaystyle{ \frac{1}{2\pi} }[/math] of this interval, pitches, it is the generalization of the common radian.

The interval has an interpretation that relates to equal divisions of all intervals. Since pitch classes in equal divisions of all intervals form in the shape of a circle, radian interval therefore occurs as the radius of this perceptional circle.

Every noticeable interval is the relative radian of some EDO of size [math]\displaystyle{ \frac{4800\pi}{7} }[/math] or smaller.