The Mayan number system is a base-twenty system that was developed by the Mayan civilization around the fourth century A.D. Only three symbols are used to represent all numeric values: a shell for zero, a bar for five and a dot for one. These symbols are arranged in columns that are read from top to bottom, and the columns are arranged in powers of 20.
Numbers one to 19 are written by combining bars and dots. For example, the number 10 is written with two bars stacked close on top of each other. The number 12 is written with two dots over two bars. For numbers higher than 19, a new row, representing multiples of 20, is added to the column of numbers. To write the number 30, a single dot is placed in the 20 row, and beneath that are the symbols adding up to 10. One multiple of 20 plus 10 equals 30. Numbers through 399 (20 x 19 x 19) can be represented in this way, so the third row in the column represents multiples of 400.
In the Mayan number system, the shell is used as a placeholder to mark a row in a column that does not have an entry. For example, the number 419 is written by placing values in the first and third rows in the column. The second row, containing multiples of 20, is marked with a shell.