IX. Post-tonal music

# Set Class and Prime Form

Brian Moseley and Megan Lavengood

Key Takeaways

- A is a group of pitch class sets related by transposition or inversion.
- Set classes are named by their .
- Prime form is the version of the set that is transposed to zero and is most compact to the left (compared with its inversion.)
- You can find prime form mathematically or by using the clock face.
- All possible set classes are summarized in the set class table, available on Wikipedia and many other websites.

# Understanding set classes

The simplest way to define is “a group of pitch class sets related by transposition or inversion.” This may initially seem confusing, but it’s just another kind of class. As you have learned in other chapters, “class” is another name for “group.” Recall the other kinds of classes you already learned about.

**Pitch vs. pitch class:**A pitch occurs at a specific octave, and often we conceive of it with a specific spelling. A is a*group of pitches*that is related by transposition or inversion.**Interval vs. interval class:**An interval has a specific distance in semitones, while an is a*group of intervals*that are inversions of each other or related by octaves.

## Groups of groups

**Pitch class set vs. set class (pitch class set class)** is the topic of this chapter. The reason this definition seems more confusing is that it involves two kinds of groups: classes and sets.

- A is a group that is related in some way.
- A

As an analogy, consider biology and the way different living things are categorized. Think of plants. A class of plants are all biologically related in a specific way. *Angiosperma* is a **class** of plants that produce flowers. But we can group together plants for other reasons: the group of plants in someone’s front yard, for example. That would be a **set **of plants, but not a **class **of plants.

So a is a group of pitches that the analyst decides to put together for some reason. The —a term that is very unwieldy, so theorists have agreed to shorten this to the last two words, —is the group of groups of pitches that are all related by transposition or inversion.

## Why transposition and inversion?

One way of analyzing at a lot of post-tonal music is by studying the transpositional and inversional relationships between pitch class sets. Take short example below: two passages from Bartók’s “Subject and Reflection” (Example 1). Comparing across the two passages, the two sets that comprise the right hand, [10, 0, 2, 3, 5] and [3, 5, 7, 8, 10], are related by T_{5}. The two left-hand sets are also related in the same way._{.}Now looking within each passage, the right and left hands are related to each other by inversion. In the first passage, they are related by I_{8}; in the second, by I_{6}.

To quickly explain why these snippets of notes all sound the same, we can say **they are all members of the same set class.**

Major and minor triads may be a helpful and more familiar example. Major and minor triads all sound “the same” in a sense. The reason for that is that all major and minor triads are transpositionally or inversionally related to one another.

- Triads of the same quality are related.
- T
_{2}of a C major triad [0, 4, 7] is a D major triad [2, 6, 9] - T
_{2}of an A minor triad [9, 0, 4] is a B minor triad [10, 2, 6].

- T
- Triads of opposite quality (major vs minor) are related.
- I
_{0}of a C major triad [0, 4, 7] is an F minor triad [5, 8, 0]. - I
_{2}of a C major triad [0, 4, 7] is a G minor triad [7, 10, 2].

- I

# Prime form

are named by their , just as pitch class sets are named by their normal form.

Note that prime form is just a label for a set class. It does not have any special status—it’s not significant if a composer uses [0, 1, 4] as a pitch class set just because it shares the same integers as the prime form (014).

## Mathematical process

Here is the process to put a pitch class set in prime form, with an example using the motive from Bartók’s “Subject and Reflection.”

Step | Example |
---|---|

1. Put the pitch class set in normal order. | [10, 0, 2, 3, 5] |

2. Transpose the set so that the first pitch class is 0. | T2 = [0, 2, 4, 5, 7] |

3. Invert the results from Step 2 and put the result in normal order. | I0 = [5, 7, 8, 10, 0] |

4. Transpose the set from Step 3 so that the first pitch class is 0. | T7 = [0, 2, 3, 5, 7] |

5. Compare the sets in Step 2 and Step 4. Whichever set is most compact to the left is the prime form. Write prime form in parentheses with no commas. |
(02357) is the prime form (Example 2). |

## Using the clock face

This video explains the differences between normal form and prime form and reviews how to find each by using the clock face.

# The set class table

There are a finite number of set classes and prime forms. Many resources have tables of these set classes, such as Wikipedia, arranged by the of the set classes. You’ll notice another number-based name for each set class in the format X–X; this is the of the set. Set class tables also pair sets together by their complements. Other features of the set class table, such as the interval class vector, are discussed in later chapters.

- Straus, Joseph Nathan. 2016.
*Introduction to Post-Tonal Theory*. 4th ed. Upper Saddle River, NJ: Prentice Hall.

- Set Class Composition prep worksheet (.pdf, .docx). Prepares students for the set class composition by asking them to find sets and transformations.
- Set Class Composition (.pdf, .docx). Builds on the prep worksheet. Asks students to compose and analyze a 24-bar ABA form piece for unaccompanied solo instrument using set classes.

A group of pitch class sets related by transposition or inversion. Set classes are named by their prime forms. E.g., (012) is a set class.

A name for a set class. The prime form is the version of the set class that is most compact to the left and transposed to begin on 0.

All pitches that are equivalent enharmonically and which exhibit octave equivalence

The smallest possible distance between two pitch classes. The largest interval class is 6, because if order is disregarded, the tritone is the largest possible interval. A P5 can be inverted to a smaller P4, m6 to M3, and so on.

In set theory, a class is a group whose members are all equivalent in some sense—transposition, inversion, octave, enharmonic, etc.

In set theory, a set is a group whose members are not necessarily related.

A group of pitch classes.

The act of moving pitch content by a certain interval.

The act of mirroring pitch content "horizontally"; i.e., so that motion down becomes up and up becomes down. Inversion often preserves intervallic content.

The number of elements in a set or other grouping.

A nomenclature for set classes developed by Forte. The first number refers to the cardinality of the set, and the second number is semi-arbitrary, but generally proceeds from the most compact to the most expanded set.