Sonata Form, Secondary Dominants & Mode Mixture

Understanding Key Features of the Classical Period

The Classical period was crucial in the development of the piano because it became the dominant keyboard instrument due to its ability to vary dynamics through touch. Composers such as Mozart, Haydn and Beethoven wrote extensively for the piano, taking advantage of its expressive capabilities.

Technical innovation and advances in the piano’s design such as improvements in range, tonal quality and responsiveness, allowed for greater artistic expression. This period solidified the piano’s role as a versatile instrument, laying the groundwork for its future prominence in Romantic music and beyond.

Understanding the specifics of a style period is invaluable for learning, interpreting and performing its music. In this blog post we look at a few key features of the Classical period, namely Sonata form and two ways in which composers enrich harmony.

Sonata Form

Sonata form (also known as sonata-allegro form or first movement form) is a musical structure commonly used in the first movements of symphonies, sonatas, and other Classical works. It consists of three main sections: the exposition, development, and recapitulation:

In the exposition, two contrasting themes are introduced, typically in different keys, establishing a tonal contrast. The development section follows, where these themes are explored, transformed and expanded through modulation and variation.

Finally, the recapitulation restates the original themes, now both in the home key, resolving the tension created earlier. Sonata form allows for a balance of contrast and cohesion, making it a key structural device for organising complex ideas within a clear framework.

Chromatic Diatonic Harmony

Using purely diatonic harmony (notes belonging to a key) makes for inoffensive, but boring music. Although not new to the Classical period, Classical composers made extensive use of chromatic harmony (using notes from outside of a key) to add colour and tension to their music.

Toward the late Classical period, especially in the works of Beethoven, the harmonic language began to expand with more frequent modulations to distant keys and chromaticism. This laid the groundwork for the more adventurous harmonic explorations of the Romantic period.

Two of the most common types of chromatic harmony that composers use within a diatonic context are the secondary dominant chord and mode mixture:

Secondary dominants

A secondary dominant chord is a dominant (V) chord that temporarily functions as the dominant of a chord other than the tonic, essentially “borrowing” the dominant function to create tension and resolution outside the home key. The third and sometimes the fifth of a secondary dominant chord are not part of the key’s diatonic scale, making a secondary dominant chord a chromatic chord. 

Secondary dominants are often used to introduce brief modulations. For example, in the key of C major, the dominant chord (V) is G major (G-B-D). The ii chord is D minor and its dominant would be A major (A-C#-E). Resolving from A major to D minor therefore introduces the C# which is not part of C major and adds a richer harmonic feel.

Mode mixture

Mode mixture, sometimes referred to as “modal borrowing” or “chord borrowing”, is when a composer uses harmonies from a parallel minor (or major) scale.  It is often used to introduce chromaticism without leaving the key entirely, adding dramatic tension or creating more expressive harmonic colours.

The most common borrowed chords in major keys are those from the minor mode, like the iv (F minor instead of F major) or the flattened VI (A♭ major instead of A minor) in C major. In minor keys, composers might borrow the I (major tonic) or the IV (major subdominant) from the major mode:

For example, in a piece in C major, a composer might borrow chords from C minor (the parallel minor) such as A-flat major to bring a rich, dark sound in contrast with the bright sound of C major. The first movement of Beethoven’s Moonlight Sonata contains extensive use of mode mixture – click here for a detailed illustration of how Beethoven uses it to expand his harmonic palette.