The complete Writing in Maya Glyphs Book 2 - Famsi

THE NUMBER 20
Before we start on the Maya calendar, there are some
special glyphs that you should know for the number 20. The
number 20 was special because the whole number system was
based on this number, and the Maya had some special signs for
20. Here are two of the most interesting, which you should learn
to recognize:
Two numbers that you will see frequently in Maya glyphs are
the numbers 29 and 30. This is because the ‘lunar month’ (i.e.
the time from one New Moon to the next New Moon) is always
about 29 ½ days. The Maya would round this fractional period to
the closest number of full days, which would be either 29 days or
30 days. Here’s how the Maya often wrote the numbers 29 and
30:
42