De Morgan's Second Theorem

The second theorem states: the complement of the PRODUCT of two logical inputs equals the SUM of their individual complements, $\overline{A\cdot B}=\overline{A}+\overline{B}$. Just as with the first theorem, this is proved by direct truth-table comparison -- computing $\overline{A\cdot B}$ and $\overline{A}+\overline{B}$ independently for all four combinations of A and B gives identical output columns in every row. Since the left side, $\overline{A\cdot B}$, is exactly the Boolean equation of a NAND gate, and the right side, $\overline{A}+\overline{B}$, is an OR gate whose TWO INPUTS are each first individually inverted (a 'bubbled OR' gate), the theorem also says a NAND gate is functionally identical to a bubbled OR gate. …