 The 20th century would see a continued expansion of acceptable practice, leading to the formulation of ideas by which there existed a "sonata principle" or "sonata idea" that unified works of the type, even if they did not explicitly meet the demands of the normative description.Sonata form shares characteristics with both binary form and ternary form.In terms of key relationships, it is very like binary form, with a first half moving from the home key to the dominant and the second half moving back again (this is why sonata form is sometimes known as "compound binary form"); in other ways it is very like ternary form, being divided into three sections, the first (exposition) of a particular character, the second (development) in contrast to it, the third section (recapitulation) the same as the first.