Sections of Haydn's piano sonatas chosen for study have been analyzed to determine the composer's technique in modulation and the key schemes resulting from these modulations, and to discover the consistent patterns and exceptional patterns in these sonata movements as a group. As revealed in this analysis, Haydn is consistent in that the key scheme patterns of several development sections are identical. Also modulations, as categorized according to key relationships, are often accomplished by the same or similar procedures. However, there are important and interesting exceptions to these consistent practices which add variety and seem to contribute to Haydn's growth as a composer