While we are all so hung up on the numeric value, conversion factors and cultural familiarity of the two systems, the essential difference between the metric and imperial units of distance measurement is often completely overlooked. So before you go and search for exactly how long it takes for light to travel between the rear legs of a gnat or how the upper arm ratio of an average Sumerian differed from that of an Ancient Egyptian, consider this.

Metric distance measurement is organised in neat multiples of ten. The common metre is composed of ten decimetres, which are in turn developed from ten centimetres constructed of ten millimetres. This is wonderful news if your main purpose in life is to manipulate measurement numerically through maths. On paper it is a doddle to add, subtract and develop the smaller units out of the larger ones. It is even relatively easy to work out the hypotenuse of a triangle. It really suits abstract problems regarding distance.

But if you haven't got a good measuring device with you, you are a bit stuffed if you want to cut a metre of cloth. You are even more stuffed if you only want 30 cm and you only have a metre mark. Of course we all carry rulers dont we, or more likely tape measures, we have to if we want to make anything.

Now it's not so long ago that a self collapsing length of spring steel was quite a specialist object, and prior to that a wooden rule was about all most people could afford. The problem with wood is that it tends to expand and contract with humidity, the longer the stick the more it would vary. so a long measure would be a bit of a liability if summer was hot and dry.

To avoid this problem you will find that a stick of wood about a foot long shows negligible variation with changes in humidity, so why not carry a stick a foot long, or 30.48 cm. No not very metric is it.

This is where the real difference between the two systems starts to become apparent.
If you have a stick one foot long that has no other markings on it, you can easily develop all of the other imperial measurements that you are ever likely to need:
Measure it out three times and you have a yard.
Halve it and you have six inches.
Halve it again and you have three inches.
Take that three inches, mark it on a piece of paper and fold it into three until the edges are tucked exactly into the two folds and you have thirded it into three separate inches.
Halve the inch and you have surprisingly arrived at half an inch.
Keep halving it again to get a quarter, eighth, sixteenth, thirty second, and a sixty fourth of an inch (0.039 mm).
If you have got a very sharp pencil you could go further down the fractions, it is possible to get to an approximation of a five hundredth of an inch in this way (a five hundred and twelfth to be precise, but the twelfth accounts for the pencil line).

Ok perhaps it sounds a bit laborious but with a bit of familiarity arriving at such an arcane measurement as thirteen and five sixteenths of an inch is not a real problem. Not as much as a problem as getting an accurate 30cm if you only have an accurate metre.
Halve it to 50, halve it again to 25?
Third it to 33.3?
Over an inch out both times.

The difference between the two systems is that the imperial system evolved through practical use on an everyday basis, it transported well and provided an appropriate degree of accuracy at every scale. It could be arrived at easily but was tricky to calculate using a decimal notation. Metric on the other hand is easily abstracted into the numerical, allowing us to perform all sorts of calculations upon it easily, as we would any other number. It really is unfriendly and to some extent counter intuitive to those who have to manipulate materials in less than ideal situations. At least it was counter intuitive to those who grew up with a system that encouraged a practical approach, as metric becomes the prevailing system we will no doubt lose that particular way of thinking.

Thanks to The Debutante for pointing out that this is also true in the kitchen "when using metric and imperial for cooking. 2oz is easier to work with than 60g. Halving or doubling recipes is a doddle in imperial. Metric gives me a headache. Okay, so 16oz in a lb and 14lbs in a stone isn't the easiest to remember, but that is, well, irrelevant" (*unless you are planing on cooking a whole person*). My italics.

A further example of the difference between numerical and measured working is hinted at in this interesting observation on medieval tailoring. Medieval clothing invariably contains odd numbers of fastenings such as buttons. This is due to the simple expedient of sewing on a button top and bottom of the opening, then sewing another mid way between the two, if three aren't enough simply split the distance between the buttons (by folding) and add more, and so on. In a world without tape measures it was very tricky to put four buttons, or any other even number, on a coat.