How did a message cross the Mongol Empire in days?

It crossed the empire by relay. Instead of one rider carrying a message for thousands of kilometres, the Mongols built the yam, also called the örtöö, a chain of stations. A messenger rode to the next station, handed the message to a fresh messenger, and rested while the other went on. Each station had food, shelter and spare horses, so nobody and no horse had to run for too long. Messengers covered 200 to 300 kilometres per day.

The stations were not far apart. Sources describe them as usually 20 to 40 miles apart, or every 25 to 30 miles, about a day's journey on horse. The idea was not entirely new: historians think the yam grew from earlier systems on the steppe and in northern China, and as the Mongols conquered northern China, the Jin posts were incorporated into it. But travellers such as Marco Polo described the Mongol version in detail, and it was unprecedented in size and efficiency.

The yam helped the Mongols govern a vast empire. It was administered by the Mongol army, and ran from Eastern Europe to the Pacific Ocean. It made it relatively easy to send important messages and travel long distances quickly. As it expanded, war routes became commercial routes: people and messages could go from Korea to Persia, or from Mongolia to Vietnam, by horse or camel caravan.

It also left a name behind. The system lasted in Russia after the fall of the Golden Horde, and the drivers of the Russian post were called yamshchiki. Today, places such as Gavrilov-Yam still carry the word in their names.