Both rounded binary and ternary form contain the following thematic scheme: A B A. Therefore, what is the difference between these two forms?
What is rounded binary form?
Binary form is a form in two parts. This form originated from 16th century European dance music, which contains repetition.
The repeat signs in binary form delineate the two parts. Three of main types of binary forms are: simple binary, rounded binary, and balanced binary. You can read more about these binary forms here.
Simple binary ||: A :|| B :||
Rounded binary ||: A :|| B A :||
Balanced binary ||: A :|| B A' :||
Figure 1. Thematic Scheme of Various Binary Forms
Rounded binary contains a thematic repetition of the opening material (A) at the end of the piece. The B section is typically short and ends with the V chord of the home key. The opening A section is usually tonally open - that means, the section begins in the tonic key, but ends in another key.
||: A :|| B A :||
Here is an example of a rounded binary.
Example 1. Mozart's Piano Sonata No.6 in D major, K.284, Third Movement
Notice the following in Mozart's example:
This excerpt contains repeat signs.
The opening theme returns in measure 13 -17.
The A section (mm. 1- 8) is tonally open. The A section ends in the key of the dominant, A major.
B section (mm. 8 - 12) is rather short - only 4 measures long.
What is ternary form?
Ternary form also contains the thematic scheme: A B A. There are two types of ternary form: simple ternary and compound ternary. For the purpose of this essay, we will focus on simple ternary because simple ternary is often confused with rounded binary.
Ternary form usually does not contain repeats signs that separate the piece into two parts.
A B A
The B section of ternary form is usually longer and more independent than the B section of rounded binary form. An independent B section usually contains its own key, own motivic ideas, and could stand alone as a small section of the piece.
The first A section is usually tonally closed - that means, the section begins and ends on the tonic.
Below is an example of simple ternary.
Example 2. Schumann, Album for the Young, "Volksliedchen"
Notice the following in Schumann's example:
This piece does not have repeat signs to divide the piece into two parts.
The opening A section returns at the end of the piece.
The B section is more stable and independent than the B section in Mozart's rounded binary. This B section is longer and clearly in the key of D major.
How do we quickly differentiate rounded binary and ternary form?
Differentiate rounded binary and ternary form by the following three parameters:
1) What is the tonal structure of the A section?
If the first A section is tonally open, it is likely to be rounded binary.
If the first A section is tonally closed, it is likely to be simple ternary.
2) Does the piece contain repeat signs?
If the piece contains repeat signs that divide the piece into two sections, it is likely to be rounded binary.
If the piece contains no repeat sign or the repeat signs do not separate the piece into two sections, it is likely to be simple ternary.
3) Does the B section sounds independent?
If the B section sounds independent (that means, the section sounds like it can stand alone as its own piece), it is simple ternary.
If the B section sounds dependent (that means, the section does not sound like it can stand alone as its own piece), it is rounded binary.
Summary on the difference between rounded binary and simple ternary:
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