On Bk. X. chap. 8, § 4 (note I, a)

After Constantine's victory over Maxentius, his half-sister Constantia, daughter of Constantius Chlorus by his second wife, Theodora, was married to Licinius, and thus the alliance of the two emperors was cemented by family ties. Constantius Chlorus was a grandson of Crispus, brother of the Emperor Claudius II., and hence could claim to be, in a sense, of imperial extraction; a fact which gave him a dignity beyond that of his colleagues, who were all of comparatively low birth. Constantine himself and his panegyrists always made much of his illustrious descent.