The operand at the top of the stack is used as the second operand for binary operators. Second from the top is used as the first operand.

Here illustrated using dc, and doing the calculation 3 - 1 and printing the answer with the command p, getting the result of 2. As we can see here with a noncommutative operation (subtraction), in order to illustrate the point, we get 2, rather than -2.

~ dc
3 1 - p
2