Salts and esters built from two phosphate units joined through an oxygen bridge, the phosphoanhydride bond that stores energy in biochemistry. Living cells from bacteria to humans run countless reactions on these compounds. Food technologists know them in more practical forms: sodium acid pyrophosphate acts as the acid side of baking powders, releasing carbon dioxide on schedule, while other pyrophosphate salts sequester metal ions, stabilize emulsified meats, and keep processed cheese smooth. Their power to bind calcium also explains their role in tartar-control products beyond the food aisle.