Sharon, a lot of things have been done over a lot of years. This is particularly true where money was an issue. The more money that you had to spend on a house, the better job you got. But in response to what you might also be asking, if your pine boards are laid diagonally, they are the underlayment. Before plywood, flakeboard and the like, pine boards were the standard underlayment for most kinds of floors including oak t&g ones.