Say you're a Godly fellow of pure heart and intention in the seventh century, and you've met a lady, and the two of you would like to have some sex. There are so many rules, though. Where does one begin?
Penitentials were handbooks carried by some priests in the Middle Ages that delineated various sins for private confession and their penances. They were full of strict limitations as to what constituted pious behavior. They went on and on. To digest it all, James A Brundage, a scholar of the Crusades, aggregated the complex rules about sex into this excellent flowchart.
Brundage originally published the chart in his book Law, Sex, and Christian Society in Medieval Europe, which is now a standard text in Medieval history, lauded on several sites like The History Blog. The book's publisher, upon learning of my appreciation for this chart, allowed me to share it here as well.
Penitentials were condemned by the Catholic church in 829, but some were still around centuries later. This sort of sex-as-infraction attitude still reverberates. Just to be safe, if you are going back in time, print it out and put it in your vest pocket. (The first question on my "Are you ready for time travel?" flowchart is "Are you wearing a vest?")