 So if you say that it's 1.04 mega roughs from here to Sydney and you tell someone that Then they're not going to know what you're talking about because they don't know what a rough is and if you tell them It's the height of a Scottish Terrier and they measure their Scottish Terrier They may well get the wrong idea because Scottish Terriers aren't very standard and there's two important things You have to have in a standard system of units The first is it has to be easy to measure and the second is it has to be the same for everybody So we all have to use the same units and over the last few hundred years people have Moaned and negotiated and tried to get towards something and we've done pretty well So now all countries in the world except for three except for Burma, Liberia and the United States of America Use what's known as the international system of units or the system in international the SI units the metric system