The Sheffer Stroke
One gate is enough for all of digital logic
The NAND gate (a ↑ b = ¬(a ∧ b)) is functionally complete on its own: every Boolean expression — AND, OR, NOT, XOR, the lot — can be built from NANDs alone. This is why entire computer chips are physically realised as a sea of NANDs. The result was settled in the 1910s by Henry Sheffer.
