Binary form

A typical example of a piece in binary form has two large sections of roughly equal duration.A piece in minor may also stay in the original key at the end of the first section, closing with an imperfect cadence.The second section of the piece begins in the newly established key, where it remains for an indefinite period of time.In rounded binary, the beginning of the B section is sometimes referred to as the "bridge", and will usually conclude with a half cadence in the original key.For example, if the A section is a period (using Caplin's naming conventions; elsewhere called a parallel period), the first ("antecedent") phrase of the A section will begin with a basic idea (generally for one half of the phrase) and end with a relatively weak cadence, and the second ("consequent") phrase will begin with the same basic idea and end with a stronger cadence; alternatively, if the A section is a sentence, the first half of the section will consist of two repetitions of the basic idea (with the second possibly transposed or slightly altered), and the rest of the phrase will drive towards a cadence.In a rounded binary form, at the end of the B section, if the A section was a sentence, typically the beginning of the sentence returns, potentially abridged, followed by the cadence; if a period, the same is true, but since the antecedent and the consequent of a period both begin the same way, it is generally simpler to say that the entire consequent phrase returns.There is some room for debate concerning how much material must return preceding the cadence; it is a subjective decision upon which theorists may differ.In rounded binary, this additional material is called a digression, and does not stand alone as a conclusive musical section, whereas in ternary, this material constitutes a complete musical section unto itself (albeit not necessarily ending on the tonic chord if an example of continuous ternary form).Putting these ideas together, if the material between the A section and the return of the thematic material does not stand alone (a digression) and only a part of the A section returns, we do not have a ternary form, and may have a rounded binary, though not all theorist would accept this terminology (and there is some overlap here with the notion of balanced binary -- see above).