Professor Forstemann's idea is that the 20 glyph changed according to its meaning and surroundings. In some cases it was represented
; while in others
or its variant
. He holds that 20 was the highest number in use among the Mayans. This he tries to show was natural enough, for, writing in The Bureau of American Ethnology Bulletin 28, p. 499, he says: "Nature suggested this ... because these they could count on their fingers and toes, in four divisions of five each." From the early historians we learn that the Mayan month consisted of twenty days (kin), which was known as one chuen (month). Any number over twenty was thus known as one chuen so many kin, or one month so many days. The Mayan year was composed of eighteen months, forming a period of 360 days known as an ahau. The month-names were (1) Pop, (2) Uo, (3) Zip, (4) Zes, (5) Zeec, (6) Xul, (7) Zo-yaxkin, (8) Mol, (9) Chen, (10) Yaax, (11) Zac, (12) Ceh, (13) Mac, (14) Kankin, (15) Moan, (16) Pax, (17) Kayab, (18) Cunku. The numbering of the years, too, was never carried beyond twenty, when it became known as a katun.[15] Thus the following table of time has been worked out:
20 kin (days) = 1 chuen (month). 18 chuen.. = 1 ahau (year) 360 days. 20 ahaus.. = 1 katun (20 years or 7,200 days) 20 katuns.. = 1 cycle (400 years or 144,000 days) 13 cycles.. = 1 great cycle (5,200 years or 1,872,000
days).73 great cycles = 1 era.