Also referred to as shukueki, they had been maintained as post stations under the system of 'Eki' (staging posts on the kaido) and Tenma (post-horses) since the ancient and medieval times, the Nara period, and the Heian Period.
宿駅ともいい、古代、奈良時代・平安時代から駅宿駅としての「駅」・伝馬の制度によって整備されていった。