The Roman History · Amm. 27.6.16

But in this affair, Valentinian went beyond the custom which had been established for several generations, in making his brother and his son, not Cæsar, but emperors; acting indeed in this respect with great kindness. Nor had any one yet ever created a colleague with powers equal to his own, except the emperor Marcus Aurelius, who made his adopted brother Verus his colleague in the empire without any inferiority of power.

Ammianus Marcellinus · translated by C. D. Yonge · 1911 reprint
C. D. Yonge translation (preface dated February 1862); G. Bell and Sons, 1911; Project Gutenberg #28587
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