The Brahmanas were served by the Kshatriyas; the Kshatriyas were served by the Vaishyas; the Shudras, adoring both the Brahmanas and the Kshatriyas, served the Vaishyas.
ब्रह्म पर्यचरत् क्षत्रं विशः क्षत्रमनुव्रताः। ब्रह्मक्षत्रानुरक्ताश्च शूद्राः पर्यचरन् विशः॥