The paper investigates an intriguing category of minor-mode sonata form: movements that tonicize the dominant in its major mode, thus proving exceptions to the generalization concerning the difference, in the minor system, between V as chord (major, with its strong resolution tendency) and as key (minor, without that destabilizing leading tone). As a key relation, such modal mismatching of minor tonic with major dominant tends to impose an artificial strain on the very nature of the minor system; for this reason it was rigorously avoided in the Classical period. It emerged as a viable alternative only in the post-Classical sonata, in which (chromatic/aesthetic) context its very problematic nature might be turned to expressive advantage. I will explore a variety of ways in which the tensions arising from the major dominant's inherent instability--expressed as a leading-tone pull towards the tonic--can affect the sonata's tonal course and middleground voice-leading basis, in extreme cases fundamentally transforming the form's very nature. Discussion of several examples from Schubert to Brahms will show how this exposition type gives rise to a range of associated tonal-formal and voice-leading categories, including the "three-part Ursatz," "failed exposition," "classicizing" modal correction within the dominant-centered S, "premature" tonic return/false exposition repeat, and (in extremis) the fully-fledged "ternary sonata" (after Jack Adrian) with structural tonic return at the start of the development—the latter represented by the first movement of Brahms's Fourth Symphony, famously characterized by Ernst Oster as a "borderline case of sonata form." The paper concludes with a more detailed analysis of this movement, where (in this and other ways) the tonicized major dominant finds its furthest-reaching structural consequences.