Also props for mentioning poor old Planty Pal.

In Aus we also had a Florin which equalled 20 pence.

We had ha'penny, penny, threepence, sixpence, shilling, florin, and a crown, which equalled 5 shillings.

So, okay, it was the bobs and crowns and guineas that I couldn't figure out. Why was there both a pound and a guinea, if they were so close in value? Was one the elder, and on its way out, while the other was on its way in? Especially considering the guinea works with odd/prime factors like 3 and 7, while the pound is a nice, likeable 2x2x5 nominator.

I mean, if you're gonna have money, have conveniently mathy money! Good, even fractions!

(M-W explains the "guinea" connection -- gold, Africa, vague, hand-wavy etymological link -- but not the logic of settling a divisible currency at an odd number.)

If you're going to have weights and measures, have conveniently mathy weights and measures!

People are just weird. There's no accounting for 'em.

IIRC, guineas were only used to price posh, luxury items--ie the price of a pair of silk gloves might be in guineas, or the account of a Harley Street doctor, but not the price of a leg of lamb. But that doesn't really explain thre reason for them.

If you're going to have weights and measures, have conveniently mathy weights and measures!

The weights and measures in the Jewish tradition (especially 'Mishna' and 'Talnud') are all based on sizes that could be measured at the time - the length of a forearm, the volume of an egg, the weight of an olive. There are endless arguments now about their conversion to current units.

In Israel, around 15-20 years ago, when the inflation was so high they had to change the currency, they didn't replace the name of the coin with a new name (like they did some years before that, when they replaced the 'Lira', which is the Hebrew word for pound, with 'Shekel'), but added "New" before it, so instead of a "Shekel" we now have a "New Shekel" ("Shekel" is a monetary unit from as early as the book of "Genesis"), or NIS - "New Israeli Shekel".

Guineas were a specific weight of gold, rather than a unit of currency. The coins were a way of saying "I (bank) confirm this coin ways X and is of Y purity".

Here's what I've found :

In 1663 the Royal Mint was authorized to coin gold pieces of the value of 20s. `in the name and for the use of the Company of Royal Adventurers of England trading with Africa'; these pieces were to bear for distinction the figure of a little elephant, and 44 1/2 of them were to contain 1 lb. troy of `our Crowne gold'. The 20s. pieces of the African company received the popular name of guineas almost as soon as they were issued, as being intended for use in the Guinea trade, and made of gold from Guinea; and the name was extended to later coins of the same intrinsic value. As silver was the sole standard till 1816, the value of the guinea was from the first subject to market fluctuations, according to the condition of the silver coin, which became so bad that the guinea rose as high as 30s. in 1695. In Dec. 1717 it was fixed at 21s., after which it underwent no further alteration. The latest coinage of guineas took place in 1813; the sovereign, of the value of 20s., was first issued in 1817.

Sounds like a scam, Nilly. Like New Coke.

Actually, come to think, USian money divides kind of funny, or used to. I've never seen a bit-coin, but they must have existed, since the song goes "Shave and a hair cut, two bits" (i.e., $0.25. A bit was twelve and a half cents.). I think that's the only circumstance under which US currency went to the 1/2 cent, though, because I'm pretty sure there's never been such a thing as a US ha'penny.

Mexico did the same thing with the peso about ten years ago - a New Peso was worth a hundred of the old, but both denominations were still in use (and may still be, I haven't been back in a while).