I bought a new computer screen. All of its specifications are in millimetres, and clearly it was designed and constructed using millimetres. However, the box proudly pronounced that the box contained a '17 inch'screen. I measured it to find that the screen is exactly 340 millimetres wide and 270 millimetres high, which looks a lot like a metric screen to me.

By calculation this suggests that the diagonal of my new screen should be about 435 millimetres or 17.1 inches. Somebody must have nicked my extra 3 mm as they were putting the screen in the box ­ I want it back! One wonders why the screen makers do this unnecessary mathematics to themselves and to us!

It's most likely because the majority of English-speaking people can visualize 17 inches much more easily than they can 435mm.

Put it down to years of mental conditioning in recognizing different size TV screens if you like, but that's the reason.