Institutio Oratoria

Again in Cicero's days and a little later, it was the almost universal practice to write a double s , whenever that letter occurred between two long vowels or after a long vowel, as for example in caussae, cassus, diuissiones. That he and Vergil both used this spelling is shown by their own autograph manuscripts.

And yet at a slightly earlier date iussi which we write with a double s was spelt with only one. Further optimnus maximus, which older writers spelt with a u, appear for the first time with an i (such at any rate is the tradition) in an inscription of Gaius Caesar. [*](Caligula, the first of the Caesars to adopt this title.)