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A yerm, a term coined by Karl Palmen for his Yerm Lunar Calendar, is an odd number n of months where almost half the months contain 29 days one more than that have 30 days ((n−1)/2 and (n+1)/2). Months are usually aligned with alternating lengths within a yerm and the first and last one are long. A mean month in a yerm therefore is 1/2n longer than 29½ days. This is useful when constructing lunar calendars, because the mean lunation is ca. 29.530589 days. The number n of months per yerm thus determines the astronomic accuracy. Best approximating a solar year of ca. 365 days, 11 and 13-month yerms have 325 or 384 days and a mean 29.(54) or 29.538461… days per month, respectively.

$\frac{29\cdot(\frac{n}{2} - \frac12) + 30\cdot(\frac{n}{2} + \frac12)}{n} = 29\frac12 + \frac{1}{2n}$