One part and binary forms

Part I - One-part forms
One-part forms comprise of one single phrase group or period that make the complete piece.  One-part forms are usually instrumental pieces and are very brief.  Characteristics of one-part form include:

1. Continuous harmonic and rhythmic movement until the final cadence
2.  Lack of a perfect authentic cadence until the end.
3. frequent use of elided cadences
4. absence of restatements of the theme in the tonic in following contrasting phrases

One-part form pieces include: etudes, preludes, where the original theme is continuously elaborated.  The Bach Prelude in F Major will serve as an example

 

You can hear that the one theme, although it modulates stays essentially unchanged throughout the entire piece.  

Part II - Binary form
Binary form looks like this:
                                        A                                               B

Fig.1 Binary form structure.

 

 There are three phrases of one theme which is repeated and three phrases of another theme which is also repeated.

In this movement, you can see all of these characteristics of binary form:

1. Repeat signs clearly divide the piece into two sections
2. The first section modulates to the dominant
3. The two sections use similar material.
4. The second section continues to elaborate material that was presented in the first section.
5. No restatement of the opening phrase of section one is heard in section two in the tonic key.