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.
A 15-month yerm has a mean month of 29.5333... days, which is too long, while a 17-month yerm has a mean month of 29.5294... days, which is too short. So a mixture of 15-month and 17-month yerms can create an accurate lunar calendar.
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.