The Teatro di San Carlo opened November 4, 1737 — 41 years before La Scala (1778), 51 years before La Fenice (1792). It's the oldest continuously active opera house in the world and the template that every subsequent European opera house copied: the horseshoe shape, the tiered boxes, the royal box on axis with the stage. Rossini, Donizetti, Bellini, and Verdi all premiered operas here. The acoustics — rebuilt after an 1816 fire in just 10 months — are considered the finest in Italy.
The interior: 6 tiers of boxes (184 total), red velvet, golden stucco, the royal box (center, enormous, crowned with the Bourbon coat of arms). Capacity: 1,386 seats. Guided tours: €9 (daily 10:30am-4:30pm, every 30 min, multilingual) — walk the stage, see the boxes, hear the acoustics explained. Opera season: October-June — ballet, opera, concerts. Tickets €30-150. Book on GYG →
Practical: Via San Carlo 98 (adjacent to Palazzo Reale, facing Galleria Umberto I). Combine: San Carlo tour → Galleria Umberto I (glass-roofed arcade, free, 1 min walk) → street food in Quartieri Spagnoli (2 min walk).