Which retail innovation, introduced in the early 1900s, fundamentally changed shopping by allowing customers to select their own merchandise without clerk assistance for the first time?

The retail landscape has evolved dramatically over centuries, with technological innovations periodically revolutionizing how merchants sell and customers shop. From ancient marketplaces to digital storefronts, retail history is filled with groundbreaking developments that changed commerce forever. This trivia question focuses on one of the most transformative retail innovations that shaped modern shopping experiences and set the foundation for today's retail environment.

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Self-service grocery stores (Piggly Wiggly, founded 1916)
Department stores with fixed pricing (Bon Marché, founded 1852)
Mail-order catalogs (Montgomery Ward, founded 1872)
Retail loyalty programs (S&H Green Stamps, founded 1896)