Klaus von Klitzing, a German physicist working at the High Magnetic Field Laboratory in Grenoble, discovers in 1980 that, when a semiconductor material is cooled to very low temperatures and subjected to an intense magnetic field, its transverse electrical resistance — the so-called Hall effect, known since 1879 — does not vary continuously and smoothly as classical physics predicts, but in extraordinarily precise discrete steps, quantized to exact values depending only on fundamental constants of nature — the electron's charge and Planck's constant — and completely independent of the sample's geometry, the specific material used, or any imperfections present in the device. This extreme precision and independence from experimental details makes the quantum Hall effect a metrological calibration tool of unprecedented accuracy: the quantized resistance has been used since its discovery as the international standard of electrical resistance, replacing earlier, less stable and precise physical standards. The quantum Hall effect also opens an entire line of research in condensed-matter physics into topological phases of matter, an area that would be considerably expanded by the later discovery of the fractional quantum Hall effect.