二段体又叫二部曲式，是乐曲形式之一，是由相对独立、结构相当、一般为同样重要的两个乐段组成的乐曲，可以作为独立的乐曲，在第一段以前也能加上序曲，在第二段以后加上结尾。也可以作为大型乐曲的基础结构。

二段体分为两种：复二部曲式和单二部曲式；单二部曲式又有主题再现式和不是主题再现两类。复二部曲式则是由两个单二部曲式组成的，或由一个单二部曲式与一个单三部曲式，甚至是其他结构的乐段组成的乐曲。

二段体可以用 A+B 来表示。

可细分为 (一)open form →continuous(第一个A非结束在I;ex:v,iii...)

Binary form is a musical form in two related sections, both of which are usually repeated. Binary is also a structure used to choreograph dance. In music this is usually performed as A-A-B-B

Binary form was popular during the Baroque period, often used to structure movements of keyboard sonatas. It was also used for short, one-movement works. Around the middle of the 18th century, the form largely fell from use as the principal design of entire movements as sonata form and organic development gained prominence. When it is found in later works, it usually takes the form of the theme in a set of variations, or the Minuet, Scherzo, or Trio sections of a Minuet and Trio or Scherzo and Trio movement in a sonata, symphony, etc. Many larger forms incorporate binary structures, and many more complicated forms (such as sonata forms) share certain characteristics with binary form.