The mail service that carried letters between Missouri and California in about 10 days from 1860 to 1861 was the Pony Express.
The system used relay stations spaced roughly 10 miles apart, allowing a rider to change horses while maintaining a rapid westward route. Riders carried lightweight mail in a special leather pouch called a mochila. The route covered about 1,800 miles between St. Joseph, Missouri, and Sacramento, California.
The Pony Express became famous for speed, but it operated for only 18 months. It was replaced after the transcontinental telegraph was completed in October 1861, allowing messages to cross the country almost instantly. A common mistake is to imagine the service as a long-running national postal system; it was a private express operation created during a brief communications gap.
The service remains one of the most recognizable symbols of the American frontier, even though it never carried passengers and depended on a carefully organized chain of stations, horses, and riders.