Self-alignment of lamellar nanodomains is accomplished by imposing thickness modulation to a block copolymer thin film. Real-time atomic-force microscopy imaging reveals that the self-alignment occurs through the directional growth of well-aligned domains along thickness gradients. This novel self-aligning principle, based on a “geometric anchoring” phenomenon, is reported by S. O. Kim, O. D. Lavrentovich and co-workers on page 2584.