If you convert the base-10 number 19,527 to base-16, you get 4C47. Here, the C is not a letter, despite still looking like one; it is a digit. Specifically, it is the digit that represents a value of twelve. Just as 7 represents a value of, well, seven. When they ran out of digits, mathematicians realized that letters were a perfect source of more symbols and already have an agreed order.
