Starting with the academic year 2005–06, a new scale was introduced, 7-trins-skalaen ("7-step-scale"; colloquially dubbed the 12-scale), designed to be compatible with the ECTS-scale: The first 0 in 00 and 02 is used to prevent fraud with grades.
The Latin scale had five steps: 0 was an outright failing grade – there was a limitation of the number of non contemendus there could be in a passing student's examination.
In 1871 the Ørsted scale was extended with plus and minus-steps, and numeral values were added accordingly.
In 1903 the Ørsted-scale was scrapped for a numeral one, with five steps: 8 and 0 were – like laudabilis præ ceteris – exceptional grades.
00 is nearly impossible to achieve, presuming one knows even a single fact taught in that particular class.
13 is a fairly rare grade outside of exams and requires a performance way beyond the expected.