Kautilya's Arthashastra 10.5.44

10.5.44

evaṃ sarvam upayogaṃ gamayet //

One-third of the best of infantry, cavalry and elephants should be kept in front; two-thirds on both the flanks and wings; the array of the army according to the strength of its constituents is in the direct order; that which is arrayed mixing one-third of strong and weak troops is in the reverse order. Thus, one should know all the varieties of arraying the array.