John Henry Freese, 1911 Encyclopædia Britannica, Volume 9… (1911)
“ Although free birth was considered indispensable, the right of wearing the gold ring (jus anuli aurei) was frequently bestowed by the emperor upon freedmen, who thereby became ingenui and eligible as equites. Tiberius, however, insisted upon free birth on the father’s side to the third generation. Extreme youth was no bar; the emperor Marcus Aurelius had been an eques at the age of six. The sons of senators were eligible by right of birth, and appear to have been known as equites illustres. The right of bestowing the equus publicus was vested in the emperor ”
