The three periods
Beethoven's compositional career is usually divided into Early, Middle, and Late periods. In this scheme, his early period is taken to last until about 1802, the middle period from about 1803 to about 1814, and the late period from about 1815.
In his Early (Classical) period, while starting out under the influence of his great predecessors Haydn and Mozart, he explored new directions and gradually expanded the scope and ambition of his work. Some important pieces from the Early period are the first and second symphonies, the first six string quartets, the first three piano concertos, and the first twenty piano sonatas, including the famous Pathétique and Moonlight sonatas.
His Middle (Heroic) period began shortly after Beethoven's personal crisis brought on by his recognition of encroaching deafness. It is noted for large-scale works that express heroism and struggle, many of which have become very famous. Middle-period works include six symphonies (Nos. 3–8), the fourth and fifth piano concertos, the triple concerto and violin concerto, five string quartets (Nos. 7–11), the next seven piano sonatas (including the Waldstein and the Appassionata), the Kreutzer Violin Sonata and Beethoven's only opera, Fidelio.
Beethoven's Late (Romantic) period began around 1815. Works from this period are characterized by their intellectual depth, their formal innovations, and their intense, highly personal expression. For example, the String Quartet, Op. 131 has seven linked movements, and the Ninth Symphony adds choral forces to the orchestra in the last movement. Other compositions from this period include the Missa Solemnis, the last five string quartets (including the massive Grosse Fuge) and the last five piano sonatas, of which the Hammerklavier Sonata is the best known.