The celebrated Quinten string quartet by Joseph Haydn, op. 76 no. 2 in D minor, is best known for its use of the perfect fifth, which permeates every aspect of the composition. However, Haydn's special use of the less apparent minor third may help us to understand how the composer conceived of the compositional process; it is the minor third that helps to guide, and perhaps even to determine, the harmonic and contrapuntal decisions that Haydn makes during the course of the quartet. The purpose of this paper is to examine the distinctive role of the minor third and to discuss how its characteristic utilization reflects a diminished seventh chord bias on Haydn's part. The unfolding of minor thirds in this quartet will be related to the issue of enharmonicism, with the most significant enharmonic relationships directly related to a principal diminished seventh sonority. While the first movement of the quartet presents all the concerns relevant to the discussion, it will be seen how each issue is presented and unfolded over the course of the next three movements as well. The fourth movement, particularly, replays each issue, presenting each within the context of its primary argument, and then resolves those issues that have become part of the very dissonant lifeblood of the quartet.