Triads in a Major Key
Harmony
In previous sections of this course, we learned about keys, scales, and chords separately. In this section of the course, we will combine these concepts in an introductory study of harmony. Harmony describes how chords interact in keys to create meaningful chord progressions. The aspects of harmony that we will consider in this course are (1) the diatonic chords in major and minor keys; (2) basic Roman numeral analysis; (3) seventh chords; and (4) cadences.
Triads in a Major Key
We will begin our study of harmony by examining the chords that occur in major keys. You can build triads on any scale degree of a major scale, as illustrated below where triads are built on each scale degree of the C major scale.
We call these diatonic chords because they are the chords that happen naturally in a key or "tonality". Click "Show Me" to hear them in the key of C major.
As we learned earlier in our discussion of white-key triads, there are three major triads (labeled M), three minor triads (labeled m), and one diminished triad (labeled d) in the C major scale.
We can name each of these triads after the scale degree of its root—both the pitch and the triad built on it share the same name. For instance, both the pitch C and the triad built on it are called the tonic: C is the tonic C major, and the triad built on C is called a tonic triad in C major, and so on with each scale degree and its corresponding triad, as illustrated below.
Triad names
If we were to transpose this C major scale to begin on another pitch, the same types of triads would occur over the same scale degrees. See, for example, the D major scale below. Like the C major scale, it has a major triad on scale degree 1, minor triads on scale degrees 2 and 3, major triads on scale degrees 4 and 5, a minor triad on scale degree 6, and a diminished triad on scale degree 7. In fact, this chord pattern is the same for all major keys (as outlined in the rule below).
| Triads in a major key |
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Roman Numerals
Music theorists use Roman numeralsA series of numeric symbols originating in ancient Rome. to identify chords within the context of key signatures. Roman numerals identify the scale degreeA single step within a scale; usually referenced by either an Arabic numeral or solfège syllable. of the chord’s root, its qualityWhen applied to an interval, the term "quality" modifies the size descriptor in order to be more specific about the exact number of semitones in the interval. When applied to triadic harmony, "quality" refers to the size of the different intervals that make up the harmony., and any extensions or inversionsChords that do not have their root in the bass voice. the chord may include. This is useful because Roman numerals convey the same information across major or minor key signatures; therefore, using Roman numerals can save time in the analysis of Western common practiceThe name that theorists and musicologists use to refer to Western music from c. 1600-1900. music.
In the following table, the leftmost column shows Arabic numerals, the middle column shows the corresponding uppercase Roman numerals, and the rightmost column shows the corresponding lowercase Roman numerals.
| Arabic Numeral | Uppercase Roman Numeral | Lowercase Roman Numeral |
| 1 | I | i |
| 2 | II | ii |
| 3 | III | iii |
| 4 | IV | iv |
| 5 | V | v |
| 6 | VI | vi |
| 7 | VII | vii |
Musicians often use the Latin alphabet letters “I” and “V” when typing uppercase Roman numerals. Likewise, they often use the Latin alphabet letters “i” and “v” when typing lowercase Roman numerals, as seen in the table above. It is important to note that Roman numerals IV/iv (4) and VI/vi (6) are often confused. It may be helpful to remember the difference by thinking of IV/iv (4) as one less than V/v (5), and VI/vi (6) as one more than V/v (5).
The triads in a major key can also be labeled using Roman numerals, as illustrated in the example below. Here, the numerals I through vii are used in place of scale degree numbers. Uppercase numerals indicate major triads (I for the tonic triad, IV for the subdominant triad, and V for the dominant triad). Minor triads are labeled using lowercase numerals (ii for the supertonic triad, iii for the mediant triad, and vi for the submediant triad). The leading tone triad is represented with a lowercase numeral and a diminished circle (viio).
Roman numerals in D major
This pattern of Roman numerals is the same for all major keys. Notice that the Roman numerals must be preceded by a key name (in this case, "D:") in order for the analysis to make sense.
The Roman numeral tells us the root, quality, and function of each triad in a key. For example, "I" in the key of D major indicates a D major triad (spelled D–F#–A), with a tonic function. A "V" in the key of C major would indicate a G major triad (since scale degree 5 in C major is G), with a dominant function. A "IV" in the key of F major would indicate a B-flat major triad (since scale degree 4 in F major is B♭), with a subdominant function. A "ii" in the key of E major would indicate an F-sharp minor triad (since scale degree 2 in E major is F#), with a supertonic function. And so on.