Denier is a measurement of fuzz weight.
It is roughly a pennyweight, depending on the weight of your penny and the heft of your fuzz. How about that?
Fine, maybe, but we can go deeper.
First off, "denier" was the name of a French coin created by Charlemagne (the old French king dude guy) in the Early Middle Ages, which, compared to now, were pretty early and long, long ago, timewise.
People liked the idea of coins so much that they stole it for other European money systems. Charlemagne though was not an original thinker — he got his idea from the earlier Roman "denarius", which was worth roughly a day's wages.
In today's money a denarius would buy around 20 dollars of stuff. Back in Roman times the basic unit of stuffness was bread and the Roman denarius was about 20 dollars worth of bread. They were big eaters in the olden days.
But back to Charlemagne, also known as "Carolus Magnus", or "Charles the Great". He was a great guy in those days, in the sense that if he told someone to pinch your head off, it was as good as done. So Charlemagne's ideas were potent, and his coinage inspired the Arab and Yugoslavian coins called "dinars". Italians called theirs the "denaro". The Spanish? "Dinero". The Portuguese, "dinheiro". Even the Republic of Macedonia has its own version, the "denar".
Ah, yes then, the British. Now we come to the British. The British were a little different. Sometimes they are. In many ways.
The British equivalent of the denier was the "penny", though the British persisted in using the letter "d" to represent it, as you might expect from them. It took 240 pennies to make one British pound, which used to be a lump of silver weighing a pound.
Are ya still with us? Fine then. We'll eventually get back to fuzz, so hang in there if you have nothing else to do.
Then the British, instead of carrying around big lumps of silver, they, the folk of the green isles, learned to fashion each lump of silver into 240 "sterlings" beginning about the year 775 (or possibly 774 ½ — no one knows for sure anymore).
"Sterlings", in case you were wondering, were silver coins based on those used by the Saxons, some early German refugees who had skipped westward across the North Sea in search of greener pastures. Some of these Saxons later got bent through various accidents and wars and things and became angled, or "Angles", which, due to interbreeding, which was common even then, is where we got the Angled Saxons, or Anglo-Saxons of today, who are now best known for the things they do involving tea.
If one of these guys had to pay off a really big gambling debt he did it in pounds of sterlings. Since they were lazy just like us, they later shortened this to "pounds sterling", and then, getting even lazier, to "pounds". And now they've gone decimal and things have gone totally to hell, though decimal numbers are easier for pocket calculators to figure out.
The original silver penny though, that was introduced by King Offa of Mercia in middle England way back when. He copied Charlemagne's denier and his coin contained about what we would call 1.5 grams of silver, to make it worth something. That amount of silver equaled a fair bit of fuzz back in the day.
So fuzz already, you may wonder, eh? Shetland cows are the cutely-kinky furry ones, with the bangs and the bushy coats and all. Also from the British Isles.
The story we're sticking with here is that intrepid knitters, during silver shortages, were able to make do by fashioning penny coins from cow fuzz, and getting them just good enough to pass as currency. King Offa's wife Cynethryth may have kicked off this trend. Let's call her "Cynthia" and avoid a bunch of lisping, which is hard to do without the guidance of a professional tutor. Cynthia it is, then.
This particular Cynthia was a wicked mad knitter, she. The weight of her fuzz coins, if she used dense fuzz, was about the same as the silver ones, which was handy, and after the conversion of the world to the metric system (except for Liberia, Myanmar, and the United Arfing States), the pennyweight became standardized at one gram. Handy.
OK for weight, but since fuzz no longer comes in tight, hefty wads and most of us have trouble running out and grabbing a cow whenever we need cash, how much is that in yarn then? We use yarn now, you know. To measure our fuzz. It is said to be a more civilized way.
Well, that would be for your 9000 meter length of yarn. (That rounds up to an even 5.59234073 miles, by the way.)
So now one denier is no longer a coin but a number representing a piece of fiber (or thread, or yarn) 9000 meters long and weighing one gram. A U.S. nickel coin is about five grams. One slim yarn there, folks.
Not tough enough all by itself to make backpacks from.
Some fabrics used in backpacks are woven from 500 to 1000 denier yarns, which means they're pretty heavy, which they need to be, to make durable-enough packs for use by clueless idiots. Stands to reason. Get your fabric heavy enough and it's even bullet proof, though everyday stuff is not 500 to 1000 denier, but around maybe 50 to 100 denier.
Thread count is another thing entirely, in case you were wondering about that. Thread count is a measure of how coarse or fine a fabric is, measured by counting the number of threads contained in one square inch of fabric, regardless of each thread's weight. (Did you notice how we just fell right back off the metric system? And landed back in the English system? Didja? We did.)
Fine quality bed sheets for example start at a thread count of 180 and go up to 250 or more threads per square inch.
So if Romans measured stuff in units of bread, then how did the British measure value in their society? (Since we seem to be stuck with them.)
Well John Heywood, a 16th century British poet once said "I shall geat a fart of a dead man as soone as a farthyng of him."
A farthing was ¼ penny, so that means a penny was worth four farts.
Who was it said that Roman civilization was the degenerate one then?
Yeeg, the British.