[perching_aix]

This is a popular argument, just one small problem with it: the 4:3 displays of old (640×480 et al) were also "wide" rather than "tall". So by this logic, there would have never been a time where horizontal tabs (or indeed, a horizontal taskbar) would have "made sense".

So I think it's reasonably easy to see that this is not and was never the actual driver behind this decision. It's completely retconned.

[layer8]

The driver was that unless you have a large number of tabs, vertical tabs waste more space than horizontal tabs, due to the width of the tabs column for vertical tabs vs. the height of the tabs row for horizontal tabs. Like in this [0] random example with just single tabs, there is a lot more wasted space on the left and right (below “My Notebook” and “Phonetics”) than on the top (to the right of “New Section 1”). If we used a vertical writing system instead of a horizontal one, we’d have had vertical tabs from the start.

Widescreen monitors afford that wasting of space better.

[0] http://www.onenotegem.com/uploads/allimg/191124/12310QH9-3.g...

[ragnese]

Well, 4:3 is less wide than 16:9 or 16:10 or whatever else we're doing these days.

But, I do agree that this was likely never the driver. In fact, I've always thought the "obvious" explanation is simply that window controls and title bars are at the top, and since tabs are like nested windows inside a window, they would follow basically the same patterns...