The history of the Overtures: when first impressions count
You never get a second chance to make a first impression! Thankfully this isn't always true, but a good start definitely matters a lot. There are many operas with famous and highly appreciated overtures and some of them maintain their position in the top ten greatest overtures ever.
The overture is simply an instrumental piece that plays before the start of the opera or one of its acts. In opera's early days, many overtures were considered incidental music that played before the audience was even seated. This was still the case during Mozart's era. Some of his most famous overtures, for The Marriage of Figaro (1786) and Don Giovanni (1787), might not have even been heard by the audience, who instead were most likely milling around chatting and eating before the official start of the opera.
However, this practice eventually changed. Starting in the early 1800s, with Beethoven and Rossini, the overture became an essential part of the opera and something to be appreciated by an attentive audience. Beethoven wrote only one opera, Fidelio (first premiered in 1805), but he actually composed four different versions of the overture until he finally had one that he deemed suitable.
Unlike Beethoven, Rossini wrote with astonishing speed, sometimes writing complete operas within a matter of weeks and frequently borrowing from himself to write his crowd-pleasing overtures. In fact, the famous tunes in The Barber of Seville's overture aren't heard in the rest of the opera. Rossini simply borrowed some melodies from two previous compositions: Aureliano in Palmira and Elizabeth, Queen of England.
Later in the 19th century, opera masters Wagner and Verdi truly elevated the overture to something magnificent, writing atmospheric introductions that also explicitly included the melodies that you were about to hear. Prime examples of this are Wagner's The Flying Dutchman and Verdi's La forza del destino, both of which basically have the same function as today's movie previews.
As the Romantic era wore on, some composers started using the term overture to indicate any stand- alone orchestral piece. In the 20th century, overtures continued to be an integral part of musical theater works, with notable examples such as the overtures to Leonard Bernstein's Candide and Rodgers & Hammerstein's Oklahoma! becoming famous in their own right.