I've been fascinated by the use of the "upright" s. It was used at the start or in the middle of a word, but not at the end, where the "modern" s was used. Double s's used two uprights if inside a word but one of each if at the end of a word. I notice several ligatures using the upright s, like st and sl. The si combination, with the top of the s replacing the dot over the i, still shows up occasionally in the fi combination in English books set in serif type. I've seen the ct ligature in books as late as the 1920s, but it always seemed like an affectation rather than the use of an obsolete standard. The similarity of the upright s and the f became a joke in Stan Freberg's historical satires when one of the Founding Fathers reads about the right to "life, liberty, and the purfuit of happineff."