Potassium sodium tartrate, known historically as Rochelle salt, is a colourless crystalline double salt of tartaric acid used as a mild laxative in medicine and as a stabiliser and acidity regulator in food. In the kitchen it appears in baking powders and effervescent preparations, where its gentle acidity activates bicarbonate leavening. It also serves as a chelating agent, binding metal ions in processed foods, and famously as the piezoelectric crystal in early microphones and guitar pickups, a curiosity of materials science. As a food additive it is considered safe, with the tartrate metabolised normally. Rochelle salt’s most famous historical role was in Fehling’s solution for sugar analysis, linking it to the analytical chemistry that underpins food quality control. It exemplifies the quiet multifunctionality of simple mineral salts.