Other comment hit the nail on the head with their link

But for those who won’t click the link, its basically just that once upon a time that’s what the price of a package of nails that size was, bigger nails cost more pennies than smaller nails.

While we’re on weird hardware measurement, I might as well talk about wire gauge

Basically it’s an arbitrary standard because it’s what someone somewhere set up their wire making equipment to do and other people just followed the same standard (though of course different parts of the world use different standards for different things, so there’s diff6 “gauge” measurements in use in various places for different things)

But the general idea is you would start with a thick wire/rod, and pull it through a die to stretch it out into progressively thinner wire

The original rod would be 1 gauge, one pass through the die and its 2 gauge, one more pass and it’s 3 gauge, etc. which is why the diameter gets smaller as the numbers get bigger

Then there’s shotgun gauge, and I have no idea why this is the standard they decided to measure this by, but it’s what it is. It’s the number of lead balls that size it would take to make a pound.

So a 12 gauge shotgun has a bore of .725 inches. It would take 12 .725 inch lead balls to make a pound.

For a 20 gauge shotgun, the bore is .615, and you’d need 20 balls that size to make a pound.

And then they throw that system out the window with .410 shotguns and just call it by the fucking bore diameter.

And I’m not gonna even touch on railroad gauges, American screw sizes, etc. not because it’s not interesting (to me at least) but because I’ve run out of fucks.

Here you go: https://www.senco.com/senco-blogs/what-is-a-penny-nail/