On his way to fight the Battle of Thermopylae, the fourth Persian King, Xerxes I, stopped in Thrace to count the men of his army. Xerxes’ army was quite large, as his infantry alone was said to have contained 1.7 million men. The king chose a particularly awkward method for counting his men: he created an enclosure that could contain ten thousand of them at a time and then marched them all through it until they had all been counted. Over a century later, another Persian king, Darius III, used the same method to count the men in his army. Ever since the Greek historian Herodotus recorded the first incident with Xerxes I, scholars have tried to understand the reason for this unusual method of counting. Scholars have speculated that the method chosen represented a difficulty with large numbers. However, both Persian kings had at their disposal several number systems able to record large numbers. These included the number systems of Mesopotamia and Egypt, both of which were well-established and capable of counting into the millions. Most likely the method amounted to a kingly boast, something along the lines of, “my army is so big, I must count the men like grain!”