Beethoven departed from the custom of his predecessors Haydn and Mozart, and the general practice of sonata-writers before him, in the matter of tonality.  In a movement in sonata-form the rule was for the second subject to be in the dominant key in the exposition section, and in the tonic in the recapitulation section, if the key of the piece was major; but if minor, in the relative major or dominant minor in the exposition, and in the tonic major or minor in the recapitulation.  Thus, if the key were C major, the second subject would be first in G major, afterwards in C major; if the key were C minor, first in E flat major, or G minor, afterwards in C minor or major.  In a minor movement the second subject is found more often in the relative major than in the dominant minor.  The first and third movements of Beethoven’s Sonata in D minor (Op. 31, No. 2) illustrate the latter; in each case the second subject is in A minor.

In major keys, besides that of the dominant, Beethoven chose the mediant (E) in his sonata in C (Op. 53); and in the recapitulation it occurs first in the sub-mediant (A), and only afterwards, in varied form, in the orthodox tonic.  Then in the B flat sonata (Op. 106) the second subject occurs in the sub-mediant (G).  In the last sonata in C minor, the second subject is neither in the relative major, nor in the dominant minor, but in the major key of the sub-mediant.  Once again, in the sonata in D major (Op. 10, No. 3) a second theme is introduced in the key of the relative minor before the dominant section is reached.  With regard, indeed, to the number of themes and order of keys, some other movements of the Beethoven sonatas show departures from the orthodox rules.

In the important matter of the repeat of the first section of a movement in sonata-form, we find the master, for the most part, adhering to the custom delivered unto him by his predecessors.  And yet there were two strong reasons why he might have been tempted to depart from it.  The repetition was a survival from the old dance movements in binary form.  E. Bach, Haydn, and Mozart not only repeated, but introduced various kinds of ornaments, and even harmonic changes; and they expected performers to do the same.  Beethoven, however, allowed no such licence—­one, indeed, which in the hands of ordinary pianists would be calculated to spoil rather than to improve the music.  Part, then, of the raison d’etre of the repeat ceased to exist.  But a still stronger temptation to suppress it must have been the programme or picture which Beethoven had in his mind when he composed.  The repeat, now become almost an empty form, must have proved at times a fetter to his imagination.  In many ways he was bold; but in this matter strangely conservative.  It was only in the sonata in F minor, Op. 57, that he first ventured to omit the repeat.  It is not to be found in the opening movements of Op. 90 or Op. 110, yet in his last sonata (Op. 111) the composer almost seems as if he wished to atone for his previous sins of omission.  He had evidently not settled the question one way or the other; but the fact that in three of his most poetical works he departed from custom, deserves note.  Before his time the repeat, like the laws of the Medes and Persians, seemed irrevocably fixed.