Mochila is the Spanish term for knapsack, although the mochilas used by pony express riders did not resemble knapsacks. Made of leather, with four pockets, or cantinas, the mochilas carried the mail. Three of the cantinas were locked. The keys were held by stationmasters at each end of the route and at the home stations where riders handed off the mail. The mochila was easy to slip on or off a saddle, and when riders changed horses, they just grabbed the mochila and swung it over the saddle of the new horse. Riders sat on the mochila-covered saddle. Openings cut into the leather allowed it to fit over the saddle horn and cantle.