Southern German varieties (SouthG) lost the preterite forms of verbs between 1450 and 1550. In spite of an abundance of attempts to explain this loss, no factor has so far been identified that explains both the timeline and areal extension of preterite loss (Fischer, Präteritumschwund im Deutschen. deGruyter, Berlin, 2018). I propose that the spread and professionalisation of merchants’ accounting in the fifteenth century was the triggering factor that led to increased use of perfects (stage 1), perfect extension (stage 2), reanalysis (stage 3) and preterite loss due to insufficient L1 input in acquisition (stage 4). The approach builds on evidence from merchant and craftsmen’s writings, a source unexplored so far. These show a correlation between a text’s topic and tenses, between profession and tense choice, and education and tense choice. The proposed analysis, finally, is consistent with the European area of preterite loss.