If you’ve ever looked at a bunk bed listing in the UK, whether it’s a single bunk bed, a double bunk bed, or a triple bunk bed, you’ve probably seen a line that says something like “maximum mattress thickness 16cm” or “fits mattresses up to 15cm deep.” Most parents read that, nod, and order whichever mattress feels nicest to lie on in the showroom. It sounds like one of those tedious manufacturer caveats. It’s not. It’s a dimensional rule with a specific origin, a specific physics behind it, and a specific kind of accident it exists to prevent. Get it wrong and the safety feature you paid for, the upper-bunk guard rail, becomes a piece of decorative wood that doesn’t do what you think it does. Here’s where the rule comes from, what actually happens when you break it, and why one university student’s life changed in 2015 because of a few inches of rail.