Why does the Mad Hatter's hat say 10/6?

The card on the Mad Hatter's hat has the two numbers 10/6, which was the price of the hat. The hat cost 10 shillings and six pence. The Mad Hatter is a character in a novel written by Lewis Carroll.

Lewis Carroll wrote "Alice's Adventures in Wonderland" in 1865. The story is about a little girl named Alice, who falls down a rabbit hole and encounters a character called the White Rabbit. While in Wonderland, Alice follows the White Rabbit and meets different characters, including the Mad Hatter. It is always tea time for the Mad Hatter in Wonderland, and the character is fond of riddles.