What does the 10/6 label on the Mad Hatter's hat mean?

The 10/6 label on the Mad Hatter's hat is a reference to the price of the hat in pre-decimal British money. It stands for "ten shillings and six pence," which is equivalent to half a guinea134.

This label was illustrated by John Tenniel in his drawings for Lewis Carroll's Alice's Adventures in Wonderland, and it has since become a significant element of the Mad Hatter's character. The date "10/6" also coincidentally matches the month and day of October 6, which is celebrated as Mad Hatter Day134.