Ludwig van Beethoven produced four versions of the overture to Fidelio. The first one is the piece now known as Leonore no. 2. For the 1806 season, Beethoven rewrote it as Leonore no. 3, now considered to be the most powerful and dramatic of the overtures to Fidelio. However, such a heavy piece overwhelmed the opening scenes of the opera, and because of that Beethoven yet again rewrote the overture into what is now known as Leonore no. 1. Fidelio was revived in 1814, and Beethoven created a new overture using fresh material, now commonly known as the Fidelio Overture, and used as the actual overture to the opera.
At some point during the 19th century, the practice arose of inserting Leonore no. 3 as a musical break between Acts I and II. This practice is sometimes attributed to Mahler, though this has been disputed.