Why do some pennies have d's?

Answer 1

The American penny was borrowed from the British currency system of pounds, shillings and pence.

The shilling and the penny were both developed all the way back in the eighth century by King Mercia of Offa, to correspond to the two main coins of Charles the Great's Empire; Charles, in turn, had adopted the names of two Roman coins, the solidus and the denarus, as he heavily used Roman terms and symbols to suggest a continuation of revival of the old Empire.

The shilling and the penny were the same size and value as their German counterparts, so right from the beginning they were occasionally referred to by the other coins' names, and the notation used to record coinage was "s" (for solidus) for shillings and "d" (for denarus) for pennies.

A penny weighed 1/240th of one pound of silver, and traders wishing to avoid the complexities of various currencies would trade in "pounds" of silver, which led to the pound currency being adopted centuries later. In English notation, this led to standard currency being referred to by the "£" (a stylised "L" for "librae" or "pounds"), "s" and "d", which was used all the way up to the 1960s in the UK, even among people with no knowledge of Latin or understanding of where the letters came from.

The traditional notation was sometimes known as the "£sd" or "lsd" system, which was obviously hilarious in the 1960s.

At rate, when the United States adopted the system of dollars and cents but chose to call the one-cent coin a penny, the old British "d" made it across.