Mozart’s piano sonata number 13, K333, composed somewhere between 1779 and 1783.
The first movement of Mozart’s piano sonata number 13, K333, composed somewhere between 1779 and 1783, (depending on the historical scholar), is a splendid, nimble piece of composition.  It is full of agility and complexity but manages to still be both very accessible and structured.  It also reflects Mozart’s admiration of and influence by Johann Christian Bach, a contemporary of Mozart’s who died in 1782.  Most musical scholars familiar with both composers agree that the opening motif of K333 bears a very flattering similarity to JC Bach’s Op. 5 no. 3 and Op. 17 no.4 piano sonatas.  Another overall source of influence is the style of the concertos of the time, grandiose and ebullient in style.  The first movement contains numerous concerto-esque flourishes that stray from the standard sonata paradigm, thereby adding an element of boldness to the piece.

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As far as the genre of piano sonatas go, K333 is otherwise fairly textbook in its construction. What is the significance of this?  The foundation of the sonata structure is the contrast, juxtaposition, and unification, of two musical keys.  Generally, these two keys are known as the ‘tonic’ and ‘dominant.’  The sonata establishes these two themes in an expository beginning section, followed by a development section in which, following the establishment of the second key, the tonality is deconstructed and its component musical segments are modified, explored, expanded upon, etc.  At the end of this development section, the piece returns to the tonic key in order to recapitulate the material from the exposition, “without the shift of key that characterized the exposition, thus harmonically unifying what was previously contrasted material.” (Edexcel, 2001) 
The sonata as a general paradigm, then, is inherently a fascinating exercise in ‘voice leading,’ or the way in which individual musical parts, or ‘voices’, interact to form chord progressions.  Individual composers were noted for their particular voice-leading styles, as each possessed particular eccentricities and tendencies when it came to tackling the challenge of creatively transitioning from chord to chord within progressions, movements, or entire pieces.  The particular rules, if you will, of the sonata were always challenges that afforded composers, e.g. Mozart, the opportunity to showcase their voice-leading creativity; specifically, how to navigate the tension and journey between the sonata’s tonic and dominant keys.  K333 is no exception and the Edexcel piece is helpful in providing a launching point for understanding Mozart’s approach to K333:
K333 is written in 4/4 time and is in the key of B-flat major; it specifically proceeds as follows, per the standard sonata conventions of the time:
Exposition: bars 1-63.Development: bars 64-93.Recapitulation: bars 94-165.
Bars 1-10 present the tonic key, which is B-flat major.  The musical theme(s) used to present the tonic key is/are known as the ‘first subject.’In bar 10, Mozart restates the opening of the piece, except transposed an octave lower.In bar 12, Mozart ‘drops’ an E-natural in the RH (right hand).  The significance of this note is that it introduces a C-major dominant 7th harmony that marks the transition of the piece to the dominant key, F-major, and is a good example of Mozart’s interesting voice-leading choices.In bars 13.4, 14.4, and 15.4, the RH sprinkles a descending group of semiquavers (also known as sixteenth notes) which illustrate that Mozart is, even early on, playfully exploring variations of the first subject – even before the introduction of the second.In bar 17.3, Mozart ‘drops’ a B-natural in the LH (left hand).  This note creates a G major dominant 7th harmony, which further punctuates the evolution away from the tonic key of B-flat major.In bar 22, Mozart accentuates the piece with an arpeggiation of a C major chord.  As the piece transitions to the key of F major, the C major chord is to become the new dominant chord; therefore, the arpeggiation is a preview or announcement of sorts of the impending transition.  This is one of many different stylistic manifestations of creative voice-leading.Bars 22-30 introduce the second subject, in the key of F major as mentioned above.  There are rhythmic similarities, however, to the first subject despite the key change.In bars 31-35, Mozart teases the listener by flirting with a transition to the key of G-minor, transitioning briefly for one bar before returning to F major and proceeding traditionally.Bar 64 commences the development section.  The quaver (eighth-note) rhythmic triads featured in Bar 1 are reintroduced and embellished upon by Mozart between bars 64 and 70.Bars 71-86 showcases Mozart’s further propensity to toy with the sonata structure by bending its rules without breaking them, per se.  He changes keys here here into F minor briefly, which is unexpectedly dramatic and concerto-like, and the right hand darts to the highest possible F note on a piano.  The darkness suggested by the F minor flirtation is explored further with brief tangents into C minor, E flat minor, and G minor between bars 75 and 86.Bars 87-90 feature an F major dominant 7th chord, which uses the chord of F to signal the imminent transition into the recapitulation section, which will take the piece back to B-flat major.Bars 94-105 feature the commencement of the recapitulation, a complete replica of the initial exposition until Mozart tosses in an A-flat chord in the RH at bar 105, continuing even in the recapitulation to introduce unusual elements.Bars 119-143 features the reappearance of the second subject in nearly identical form as its original presentation, except transposed to the tonic key.Bars 152-165 comprise the coda, which is mostly a duplication of bars 50-63 of the exposition with some flourishes thrown in, showing Mozart refuses to completely adhere to structure for structure’s sake.
What is pleasurable about the piece overall, then, is clear.  Mozart is faithful to the stylistic requirements of the genre, not deviating from the overall structure, while being creative with the execution of the interplay between the two keys he employs.  Furthermore, Mozart’s voice-leading technique is bold and enhances the piece’s complexity, particularly the mood-darkening introduced by his transitions into minor keys, including F minor, and also in his methods of announcing transitions between keys by flourishes or arpeggios.
Finally, it is worth mentioning that other technical elements, though perhaps unmoving to an expert, are nonetheless astonishing to a layperson or non-musician.  The tempo Mozart utilizes is astonishingly brisk given the acrobatics required by the composition.  To appreciate the piece is also to appreciate the skill necessary for a person to perform it competently, much less expressively.
BIBLIOGRAPHY
Mozart, Wolfgang Amadeus.  Piano Sonata No. 13 in B flat, K.333, circa 1781
“Mozart: Piano Sonata in B-flat K333, first movement” in Edexcel A Level Syllabus – Analysis [book on-line] (Edexcel, 2001, accessed 11 June 2005); available from http://www.musicteachers.co.uk/resources/k333.pdf