structural:reformatting, stylistic:tone
Romantic period

In the early 19th century, the current usage of the term sonata was established, both as regards form per se, and in the sense that a fully elaborated sonata serves as a norm for concert music in general, which other forms are seen in relation to. From this point forward, the word sonata in music theory labels as much the abstract musical form as particular works. Hence there are references to a symphony as a sonata for orchestra. This is referred to by William Newman as the sonata idea.

Among works expressly labeled sonata for the piano, there are the three of Frédéric Chopin, those of Felix Mendelssohn, the three of Robert Schumann, Franz Liszt 's Sonata in B minor, and later the sonatas of Johannes Brahms and Sergei Rachmaninoff.

In the early 19th century, the sonata form was defined, from a combination of previous practice and the works of important Classical composers, particularly Haydn, Mozart, Beethoven, but composers such as Clementi also. It is during this period that the differences between the three- and the four-movement layouts became a subject of commentary, with emphasis on the concerto being laid out in three movements, and the symphony in four.

Ernest Newman wrote in the essay "Brahms and the Serpent":