In defense of slide rules

I own two slide rules and use neither for anything real, which is the correct number and the correct amount of use. But I keep them on the desk, because a slide rule has one property no calculator has: it refuses to tell you the order of magnitude. It gives you 2.9, and whether that means 29 or 2,900 is your problem. You cannot operate one without holding a rough answer in your head first.

That constraint — estimate first, compute second — is the single most useful habit I've stolen from any tool. The Wikipedia article is a good rabbit hole if you want the mechanics; the short version is that it's a log table you can touch.