Classical era style (c.1750-1820) didn't suddenly appear during the mid-eighteenth century, but evolved gradually with traits of the late Baroque mixing freely with those of the emerging Classical style. We will focus on characteristics of the fully formed Classical style as exemplified in the First Viennese School.
Lady and Lyre Guitar | Pietro Nocchi, 1783-1855 | Bowes Museum
The First Viennese School refers to composers of the Classical era in late eighteen century Vienna, primarily Wolfgang Amadeus Mozart, Joseph Haydn and Ludwig van Beethoven. They were the most important composers of this era—the pinnacle of Classical style—and influenced generations of composers into the nineteenth century.
Classical Style
Melody
At the most basic level, Classical melodies revolve around the technique of contrasting a melody with a second melody. Question and answer phrases, also called antecedent and consequent phrases, are a typical Classical era method of creating melodic contrast. For example, the two phrases of "Row, Row, Row the Boat" are antecedent and consequent phrases.
Row, Row, Row The Boat | Antecedent and consequent phrasing
Line 1 is the antecedent phrase (the question) due to use of an incomplete cadence (unstable note) at measure 4. Line 2 is consequent phrase (the answer) due to use of a complete cadence (stable note) at the end.
Classical melodies have shorter and more symmetrical phrases than prior eras. The four-measure phrase is the norm. Two to four phrases were organized into a set known as a period and concluded with a strong cadence. The two four-measure phrases of "Row, Row, Row the Boat" comprise a period.
Melody is dominated by smooth and lyrical melodic lines. These melodic lines are often based on chord structures. The Star-Spangled Banner and Twinkle Twinkle Little Star are melodies based on chord structure. Ornaments (decorative notes) such as the turn, trill and appoggiatura were used extensively and form an integral part of the melody. Here's a typical Classical melody:
L'Encouragement (excerpt) | Fernando Sor | Frary Guitar Duo
A Musician and his Family, possibly Fernando Sor | France, nineteenth century (anonymous) | Bibliothèque Paul-Marmottan
Harmony
The Baroque practice of basso continuo faded away during the Classical era. Compared to the Baroque, Classical chord progressions are simple and slow moving. Textures are predominantly homophonic.
Alberti Bass
To drive a piece rhythmically, Alberti bass was often employed. Alberti bass is an arpeggiated accompaniment where chord tones are sounded in order of lowest, highest, middle, highest, usually in patterns of eighth or sixteenth notes.
Alberti Bass | F. Carulli, Duo in G Major Op. 34 No. 2: Largo | Frary Guitar Duo
Modulation
Key changes—modulation—was used extensively to create tension and contrast. Modulation is linked to formal structure and enhances drama in longer pieces.
Musical works increased in length during the Classical era. Thus, formal structure became increasingly crucial as an element of coherence. Symmetry and balance were important in the grouping of musical ideas. Forms are precise and clear, with sections marked off by distinct cadences.
Ternary Forum
Formal structures such as binary form, ternary form, and theme and variation were common in the Classical era. Ternary form, also called A B A form, was especially significant in instrumental music.
Andantino in A Minor uses ternary form: outer A sections frame the inner B section. The A section is cast in the key of A minor and, beginning at 00:26, the B section shifts to a sunny C major. The dark A section returns at 00:42.
Andantino in A Minor | Ferdinando Carulli, 1770-1841
Binary Form
The example below, Sor's Variation 1 from Duo Opus 55, No. 3, uses four-measure phrases and clear-cut periods with distinct cadences. Sections are arranged in an A A B B sequence, formally known as binary form. Harmony is slow moving, usually changing chords once every two beats.
Duo Opus 55 No. 3: Variation 1 | Fernando Sor, 1778-1839 (1:23)
Rests and Rhythm
Classical rhythm is straightforward compared to the Baroque and organized into short patterns punctuated by rhythmic cadences. Rests—periods of measured silence—are a common dramatic device after a strong cadence:
Dramatic Rest | F. Carulli, Duo in G Major Op. 34 No. 2: Largo | Frary Guitar Duo
Movements
Instrumental pieces typically consist of several movements of contrasting character and tempo. For example, the movement sequence in the sonata, symphony and string quartet follow a four-movement sequence of fast-slow-dance-fast. The concerto of the eighteenth and nineteenth centuries normally employ a three-movement format of fast-slow-fast.
Dynamics
Dynamics were used more frequently and with finer gradations than the Baroque era. The crescendo and decrescendo were hallmarks of the style.
Unlike the single mood of Baroque movements, Classical works fluctuate, often between extremes: turbulence to restfulness, steadfastness to playfulness, happy to sad, etc. Mood contrasts were illustrated with sudden tempo changes, mode changes (e.g., major to minor) and dynamic contrast. The example below, Rondo Op. 34, shifts from a bright and happy major key to a dark and serious minor mode. It also uses a rest to enhance the drama of the mood change:
Rondo, Op. 34 (excerpt) | F. Carulli | Frary Guitar Duo
A Musical Party | Vincenzo Vita, d. 1782 | Home music making was an indicator of higher educational and class status. | Bowes Museum
Vocabulary
First Viennese School, ornaments, Alberti bass, modulation, rest, period, question and answer phrases, antecedent and consequent phrases
Classical Style
Melody
Harmony
Formal Organization 
Ternary Forum
Binary Form
Rests and Rhythm
Movements
Dynamics
Mood Contrasts
