https://en.wikipedia.org/wiki/Guinea_(coin)
"Even after the coin ceased to circulate, the name guinea was long used to indicate the amount of 21 shillings (£1.05 in decimalised currency). The guinea had an aristocratic overtone; professional fees and payment for land, horses, art, bespoke tailoring, furniture and other luxury items were often quoted in guineas until a couple of years after decimalisation in 1971.[9]"
Copper (pennies) for retail purchases. Silver (shillings and pounds) for wholesale. And gold (guineas) for finance (and government) transactions. Kind of.
That's partially based on my reading of Adam Smith (who spends a tremendous amount of time on coinages) and partially my own speculation, though it seems to have at least some merits.
Keep in mind that pennies went down to the farthing (a quarter penny).
For labourers (ag, day, general, messengers, miners), annual wages were ~17 - 31 pounds/yr, roughly, as of 1700.
For some historical wages and costs: