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The ancient Maya civilization used a roughly vigesimal (base-20) numeral system.
The numerals are made up of three symbols; zero (shell shape), one (a dot) and five (a bar).
For example, 19 is written as four dots in a horizontal row above three horizontal lines stacked upon each other.
Digits are stacked with the higher significant digits at the top. Thus, two dots above each other would be read as 1×20 + 1 = 21. Alternatively, the numerals can be written side by side, in which case, the higher significant digit is written on the left, as in our number system.
Most of what we know about Mayan numbers comes from a single document, the Dresden Codex which was a treatise on astronomy and the Maya calendar. Throughout this document, a mixed base-18/base-20 number system is used. The third numeral from the bottom (or from the right) does not represent 20×20 = 400, as would be expected, but 18×20, so that one dot over two zeros signifies 360. This is supposed to be because 360 was an early approximation to the number of days in the solar year. Subsequent place values return to base 20, representing 20 times the previous digit, so the fourth, fifth and sixth digits represent 20×18×20 = 7,200, 20×20×18×20 = 144,000 and 20×20×20×18×20 = 2,880,000 respectively.
There is no evidence that a true base-20 system was used by the Mayans, but the mixed base system that we know they did use seems so cumbersome that many scholars of the Mayans feel that they must have used a pure base-20 system in their day to day life and that the mixed base system was a special one used only by the priests and astronomers and for recording dates. This may be just wishful thinking.
It is thought that the dot for a unit represented a cocoa bean, since the Mayans used cocoa beans as money, but it is also possible that it represents a pebble.
Other than the bar and dot notation, Maya numerals can be illustrated by face type glyphs.
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