When I was 18, I worked on a summer camp in the US (as I have intimated before - a "little camp in Pinola, MS"). The camp director recruited a range of people from Europe to work as counsellors and kitchen staff, alongside US children about to go to university. But that's not important right now. What interested me was the fact that, when I was asked my weight, and replied using the conventional English units (I think I was around 9 stone at the time - that's 128 pounds) I was considered unbelievably quaint by the US staff ("Stones? What kind of measurement is that?"). For those reading this who don't know, 1 stone is 14 pounds. I know, I know - don't even go there.
So what? Well, I am pretty au fait with a range of units. I am as happy in litres of milk as pints of milk. I buy meat and vegetables in kilos, not pounds and ounces. I buy petrol and diesel in litres. I can work interchangeably in Celsius and Fahrenheit from 0 degrees C to about 30 degrees C (that's 32 degrees F to 85 degrees F). But I can only tell you the birth weights of each of my three children in pounds and ounces - not in kilogrammes, which was the actual unit of measurement used, and what was written down in their health records. Why is that?
I suspect it's because, whereas most units relate to something you are getting "now", the birthweight of a child will always be compared to something that has happened some time ago - whether it is the baby of a mum in the NCT group that was born last week, or the weights of grandma's children. As long as birthweights are being related to something historical - which will be forever - I suspect they will have to use "legacy units". We got the impression that the medical staff really took seriously their obligation to deliver this in metric units, but were also incredibly conscious of the fact that these units meant nothing to anybody.
Another unit like this is "miles per gallon" - built-in computers on cars continue to make this available in the UK, despite the fact that you haven't been able to buy a gallon of petrol for years. But the proportion of people for whom the metric "litres per 100 km" means anything must be vanishingly small.