Whisky made with backset added during the initial mash. Whiskies using backset include bourbon, corn whiskey and Tennessee whiskey. Backset, also known as backset stillage adds a lot of character to whisky and also promotes consistency across batches.
Backset adds acidity to the wort which was historically beneficial for limiting unwanted bacteria. Sour mashing is also a throwback to the economic practices employed by early day moonshiners. Regulations stipulate that a sour mash contain at least 25% backset. See Title_27.