Rounded binary form is a musical form similar to simple binary form except that the second section is often longer than the first and part of the first section is repeated at the end of the second. It resembles ternary form in that it has a recapitulation of the opening material but differs in that such restatement is usually only partial and the contrasting section is not as substantial. Rounded binary form was employed extensively in the Classical era and was frequently used as the structure for the theme in a set of theme and variations. This form was a precursor of sonata form and was employed extensively in piano sonatas including those by Mozart and Beethoven.