Used to describe a timeless work of art, the term "classic" refers to something that is exemplary, easy to understand, and conforms to an ideal of beauty. It applies both to the entire genre, as well as the epoch of classical music.
Bach's sons and Leopold Mozart are representatives of the so-called pre-classical period, characterised by the refined and sensitive style and expressive musical language of the Mannheim School and the First Viennese School. This early phase of the classical period paved the way for Viennese Classicism, with its three protagonists Joseph Haydn, Wolfgang Amadeus Mozart, and Ludwig van Beethoven. Moving away from complex polyphony, melodies began to take over and tonal, major/minor harmony that had begun in the Baroque period, became just as important to the music, as the support for the melody. The sonata form, found in sonatas, symphonies, and other chamber music genres of the Viennese Classical period became typical. The capital city of Vienna helped breed the creativity of the era as the home of many important and generous patrons, including Prince Nikolaus Esterházy, Baron Gottfried van Swieten and Rudolf, Crown Prince of Austria.
With several innovations and revolutionary ideas, the instruments were also undergoing changes. The harpsichord was slowly replaced by the piano, allowing performers to play more softly and subtly. Instruments like the clarinet, flute, horns, and oboe were also improved and became more versatile, granting them their own section in the traditional symphony orchestra, which in turn grew and cemented into what we now know as an orchestra today. The string quartet also came into its own.
Symphonies, concertos, solo instrumental pieces, piano sonatas, and operas expanded massively and the spirit of the Enlightenment was often reflected in the music in lilting compositions set in major keys. As successful musical experiments continued, the Classical era saw delicate sounds become intertwined with emotion and indulgence. It was Beethoven, however, who forged the path into the next musical period, and while the Romantic era had already begun, it wasn't until Beethoven's death in 1827, along with that of Franz Schubert in 1828, that the Classical era came to an end.