The first movement of Mozart's Sonata No. 1, K. 279, is a lively Allegro in C major. Short chordal statements, running sixteenth notes, and quick exchanges between the hands give the music a conversational character. The right hand often carries the melody, while the left hand alternates between broken-chord accompaniment, short answers, and more active running figures. Trills, grace notes, and frequent changes between loud and soft playing add detail to the movement's clear phrases.

Measures 1-16: The opening places a firm right-hand chord against a brief rising bass figure before the upper hand begins its run. This chord-and-run gesture returns immediately. A quieter melody then appears above continuous left-hand sixteenths, with short slurs and rests separating the upper phrases. Later exchanges introduce detached notes, small ornaments, and changing registers. Near the end of this section, held right-hand chord tones support an independently moving upper voice, followed by short trills and a rising and falling scale.

Measures 17-38: A decorated upper arrival is paired with a quick left-hand ascent. The music develops this contrast between a sustained melody and moving accompaniment, then introduces dotted upper phrases, repeated broken-chord patterns, and further trills. Some figures pass between the hands; others leave one hand briefly silent. In the closing material, the right hand moves through changing contours and articulations above lower chordal responses. Longer held notes and grace-note figures lead to the exposition's closing repeat sign at measure 38.

Measures 39-57: The development brings back the opening chord-and-run idea in changing harmonies. Accidentals alter the keyboard pattern as the figure moves through new positions. The hands briefly carry different moving lines before the left hand returns to a more continuous broken-chord texture. Short upper trills alternate with quieter phrases. Later measures exchange lower running figures and upper chordal statements, building toward a right-hand run that prepares the return of the opening.

Measures 58-81: The principal idea returns in C major. Familiar material continues with altered harmonies and different register choices. The left hand's broken figures move between positions beneath a more spacious upper melody. Held chords and ornaments again create several layers within the right hand. Broader ascending and descending runs lead into dotted phrases, trills, and triplet figures, with the hands maintaining different patterns of movement and release.

Measures 82-100: Coordinated broken figures lead into the final group of trills and upper ornaments. The right hand revisits wide contours and detached notes while the left hand supplies short figures and chordal replies. The closing passage drops the right hand into the bass register before it returns upward. A final ornamental ascent leads to rolled upper chords over a compact left-hand figure and a clearly separated ending.

Interesting fact: The final page briefly writes the right hand in bass clef. Reading the changing clefs reveals how far the upper part descends before the closing return to the treble register.