Reading Palladio on the flight home (because, really, why wouldn’t you). His Quattro Libri have been excerpted to no end, and there are familiar passages that most course readers focus on (mine included). But in the bits that usually get passed over, specifically the chapter with the fetching title of “Of stairs, and the various kinds of them; and of the number and size of the steps,” there’s this, which could have been pulled right from the International Building Code:
THE ſteps ought not to be made higher than ſix inches of a foot; and if they are made lower, particularly in long and continued ſtairs, it will make them the more eaſy, because in riſing one’s ſelf the foot will be leſs tired ; but they muſt never be made lower than four inches: the breadth of the ſteps ought not to be made leſs than one foot, nor more than one and a half.
THE antients obſerved to make the ſteps uneven in number, that beginning to go up with the right foot, one might end with the ſame ; which they look’d upon as a good omen, and of greater devotion when they entered the temple: The number of ſteps is not to exceed eleven, or thirteen at moſt, before you make a floor or reſting-place, that the weak and weary may find where to reſt themſelves, if obliged to go up higher, and be able more eaſily to ſtop any thing that ſhould happen to fall from above.
OK, not so much about the uneven steps, but with some minor adjustments to take into account the difference between a Vicenzan foot and an Imperial one, thirteen steps is still, five hundred years later, the maximum number of risers between landings for precisely those reasons; and he’s got the tread to riser ratio right in the sweet spot as well.