Shilling

The shilling was a form of currency used in Great Britain up through the mid 20th century. Its value was 20 shillings to a pound sterling, and 12 pennies to a shilling. Susan Foreman, being a time traveler, had visited Britain's future before, and, simply being confused, could not remember how many shillings were in a pound when she and the First Doctor settled down in 1963 London.

It would only be after 1971 that Great Britain would switch to the decimalised Pound, with the shilling now at 5 new pence. Susan probably didn't remember which year it was that this occurred, resulting in her ridicule when she thought that they had already decimalised the currency while attending high school. (DW: An Unearthly Child)


 * Interestingly, with An Unearthly Child set and broadcast in 1963, Susan accurately predicts that England would switch to decimal currency, which in the real world was announced in 1966 and occurred in 1971.