Most music uses a variety of different chords. This piece is in C major. The first beat of bar 1 uses the notes from the C major tonic triad. We can show this by writing first the key, C major, then a capital I for the tonic triad.
The second beat uses the notes from the G major triad. In the key of C major, this is the dominant triad, so at the place where the chord has changed, we can write in a capital V for chord 5.
In bar 2, all the notes belong to chord I, the tonic triad. In bar 3, all the notes belong to chord V, the dominant, and in the last bar, the two notes belong to the tonic triad. We haven’t used all the tonic triad notes in the last bar, but that doesn’t matter, as we’ve used the most important two – the root and the third of the triad.
Now we have list of chords written in the order they appear in the music: I-V-I-V-I.
A list of chords in the order they appear, using Roman numerals, is called a chord progression.
In earlier grades we learned about cadences and pre-cadence chords. The cadence chord progressions are all very common, whether they are used at a cadence or elsewhere in a piece of music. You will frequently find the progressions V-I, IV-I, and a variety of chords moving into chord V.
A chord progression is a series of chords which make up a piece of music. Although there are lots of chords to choose from, in practice some chords go together better than others and other chords are rarely used together. Some chord progressions are common in one style of music, but very unlikely in another.
When we learn about harmony and chords, we tend to start off by looking at the progressions used in Baroque and early Classical music written in Western Europe (for example those used by Bach and Mozart). This is because music at that time followed fairly strict rules, and the chords that were used were more limited than those used in later periods. This makes it easier for a beginner to understand – the “rules” are clear, and most music from the era follows the rules. Most of the terminology relating to chords and harmony belongs to this period.
All the rules that are explained in this section of the course relate to the style of Classical harmony, which is also the style you are tested on in theory exams. As musical styles evolved through the Romantic period (19th century) and into the modern era, many (perhaps all!) of these rules have been abandoned, so it is quite likely you will have heard or played music which does not follow these guidelines.
The harmony of the later periods is more complex, and well worth studying after you have a solid understanding of the earlier styles, but for the later period there are no “rules” as such, and individual composers from the same era can differ considerably.
It’s also important to mention that many musical styles such as jazz, rock, pop, folk and music from other parts of the world (such as Africa and Asia) does not necessarily follow the same conventions.