Beethoven is viewed as a transitional figure between the Classical and Romantic eras of musical history. Above all, his works distinguish themselves from those of any prior composer through his creation of large, extended architectonic structures characterized by the extensive development of musical material, themes, and motifs, usually by means of "modulation", that is, a change in the feeling of the home key, through a variety of keys or harmonic regions. Although Haydn's later works often showed a greater fluidity between distant keys, Beethoven's innovation was the ability to rapidly establish a solidity in juxtaposing different keys and unexpected notes to join them. This expanded harmonic realm creates a sense of a vast musical and experiential space through which the music moves, and the development of musical material creates a sense of unfolding drama in this space. In this way Beethoven's music parallels the simultaneous development of the novel in literature, a literary form focused on the life drama and development of one or more individuals through complex life circumstances, and of contemporaneous German idealism's philosophical notion of self, mind, or spirit that unfolds through a complex process of contradictions and tensions between the subjective and objective until a resolution or synthesis occurs in which all of these contradictions and developmental phases have been resolved or encompassed in a higher unity.
Beethoven continued to expand the "development" section of works, extending a trend in the works of Haydn and Mozart, who had dramatically expanded both the length and substance of instrumental music. As Beethoven's major immediate predecessors and influences, he looked to their harmonic and formal models for his own works. However, both Mozart and Haydn placed the great weight of a musical movement in the statement of ideas called the exposition, for Beethoven the development section of a sonata form became the heart of the work. Beethoven was able to do this by making the development section not merely longer, but also more structured. The very long development section of the Eroica Symphony, for example, is divided into four roughly equal sections, making it, in effect, a sonata form within a sonata form. The first movement alone of this symphony is as long as an entire typical Italian-style Mozart symphony from the 1770s. His focus on the development would, like others of his innovations, set a trend that later composers would follow.
Although Beethoven wrote many beautiful and lyrical melodies, another radical innovation of his music, compared especially to that of Mozart and Haydn, is his extensive use of forceful, marked, and even stark rhythmic patterns throughout his compositions and, in particular, in his themes and motifs, some of which are primarily rhythmic rather than melodic. Some of his most famous themes, such as those of the first movements of the Third, Fifth, and Ninth symphonies, are primarily non-melodic rhythmic figures consisting of notes of a single chord, and the themes of the last movements of the Third and Seventh symphonies could more accurately be described as rhythms rather than as melodies. This use of rhythm was particularly well suited to the primacy of development in Beethoven's music, since a single rhythmic pattern can more easily than a melody be taken through a succession of different, even remote, keys and harmonic regions while retaining and conveying an underlying unity. This allowed him to combine different features of his themes in a wide variety of ways, extending the techniques of Haydn in development (see Sonata Form).
He also continued another trend - towards larger orchestras - that went on until the first decade of the 20th century, and moved the center of the sound downwards in the orchestra, to the violas and the lower register of the violins and cellos, giving his music a heavier and darker feel than Haydn or Mozart. Gustav Mahler modified the orchestration of some of Beethoven's music -- most notably the 3d and 9th symphonies -- with the idea of more accurately expressing Beethoven's intent in an orchestra that had grown so much larger than the one Beethoven used: for example, doubling woodwind parts to compensate for the fact that a modern orchestra has so many more strings than Beethoven's orchestra did. Needless to say, these efforts remain controversial.
In his Fifth Symphony Beethoven introduced a striking motif, drawn from a late Haydn symphony, in the very opening bar, which he echoed in various forms in all four movements of the symphony. This is the first important occurrence of cyclic form.
He was also fond of making usual what had previously been unusual: in the Fifth Symphony, instead of using a stately minuet, as had been the norm for the "dance" movement of a four-movement work, he created a dark march, which he used as the third movement and ran into the fourth without interruption. While one can point to previous works which had one or more of these individual features, his music, combined with the use of operatic scoring that he learned from Mehul and Cherubini, created a work which was altogether novel in effect - too novel, in fact, for some critics of the time. On the other hand, his contemporary Spohr found the finale "too baroque", though he praised the second movement as being in "good Romantic style".
His Ninth Symphony included a chorus and solo voices in the 4th movement for the first time, and made extensive use of fugues, which were generally considered to be a different form of music, and again unusual in symphonies.
He wrote one opera, Fidelio. It has been said that he wrote beautiful vocal music without regard for the limitations of human singers, treating the voice as if it were a symphonic instrument - even though his conversation books note his desire to make his music singable and include references that indicate that he had remembered his father's singing lessons.
Beethoven's development and works are typically divided into three periods: an early period in which his works show especially the influence of Mozart and Haydn; a middle, mature period in which he developed his distinctive individual style, sometimes characterized as "heroic"; and a late period, in which he wrote works of a highly evolved, individuated, sometimes fragmented and unorthodox style sometimes characterized as "transcendent" and "sublime", where he tried to combine the baroque ideas of Handel and Bach with his icons Mozart and Haydn. In his late years he called Handel "my grand master".
In contrast to Mozart, he labored heavily over his work, leaving intermediate drafts that provide considerable insight into his creative process. Early drafts of his Ninth Symphony used rough vertical marks on the score in place of actual notes, to indicate the structure he had in mind for the melody. Studies of his sketch books show the working out of dozens of variations on a particular theme, changing themes to fit with an overall structure that evolved over time, and extensive sketching of counter-melodies.