Used by some ancient Greeks, it was made up of a year of twelve months of twenty-nine or thirty days each, plus an intercalary month of twenty-nine or thirty days. This extra month was placed after the sixth month every two years. Because it made the year too long, it was occasionally omitted, finally added on an irregular basis.
Each month was divided into three ten-day periods. For the first two periods, the days were counted in forward sequence. In the third period, they were counted in backward sequence from the last day of the month.
