Why is a television measured diagonally?
Because the screens used to be round.
They get to advertise a bigger number.
It’s a single standard number that gives you a sense of the screen size. If they went with height or width, they’d have to tell you which it is so there’s no uncertainty (or give both). The diagonal value is unambiguous and simple and the quickest way to convey the size with the least amount of information.
Well, it used to be unambiguous when all TV screens were the same aspect ratio.
That it started because tv screens were round is a standard answer. The big problem is that it’s almost impossible to find a circular screen on a commercial television in the 1940s or early 50s.
The circular screens were standard on oscilloscopes, a scientific instrument only found in labs. Some very early televisions also used circular screens because they already existed. Saying that 1930s experimental tv’s influenced store sales twenty years later is too much of a stretch for me.
Doing a simple Google image search for television sets 1950 reveals that almost every set had a rectangular screen with rounded corners. Without the straight lines of the box that the tv came in, producing a set of numbers that were comparable across all the dozens of styles was a marketing nightmare. Much easier to do a diagonal as the single public number even if that made little sense going from a spot on a curve to another spot on a curve. More to the point, that was the biggest possible number to tout. Bigger always wins, except in golf and Ozempic.
Nitpick: It’s true that rectangular picture tubes were introduced in the 1940s, but they weren’t perfected for color TV until the 1960s. The first mass produced rectangular picture tube for color TVs wasn’t introduced until late 1963. Until then color TV screens were framed to provide a rectangular picture that was by then the standard look for black and white.
I’ve done a lot of reading on that Early Television site; fascinating place.
That tidbit on color tv screens is true but also irrelevant. The dealer sold what showed to the customer, not the tube itself. Television had been around for years before the first commercial color sets were marketed; the FCC dithered between the CBS and NBC systems until 1953 and the first sanctioned broadcast was the Tournament of Roses parade on NBC on January 1, 1954. Advertising sizing existed long before 1963 and long before the first commercial color sets.
Moderating: I’ve changed the title to be more descriptive.