The Real Teatro di San Carlo (“Royal Theatre of Saint Charles”), as originally named by the Bourbon monarchy but today known simply as the Teatro (di) San Carlo, is an opera house in Naples, Italy, connected to the Royal Palace and adjacent to the Piazza del Plebiscito. It is the oldest continuously active venue for opera in the world, having opened in 1737, decades before either Milan’s La Scala or Venice’s La Fenice.
Commissioned by the Bourbon King Charles III of Naples (Carlo III in Italian), Charles wanted to endow Naples with a new and larger theatre to replace the old, dilapidated, and too-small Teatro San Bartolomeo of 1621, which had served the city well, especially after Scarlatti had moved there in 1682 and had begun to create an important opera centre which existed well into the 1700s.
Thus, the San Carlo was inaugurated on 4 November 1737, the king’s name day, with the performance of the opera Domenico Sarro’s Achille in Sciro, which was based on the 1736 libretto by Metastasio which had been set to music that year by Antonio Caldara. The first seasons highlighted the royal preference for dance numbers, and featured among the performers famous castrati.
The opera season runs from late January to May, with the ballet season taking place from April to early June. The house once had a seating capacity of 3,285 but has now been reduced to 1,386 seats. Given its size, structure and antiquity, it was the model for theatres that were later built in Europe.
According to Wikipedia