sour milk???

I was looking thru my Great-Grandmother’s recipes and one of the recipes calls for sour milk. How do you get sour milk? Thank you for your help.

sour milk is buttermilk, or you can make sour milk with 1 tsp lemon juice or vinegar to a measuring cup and then fill to one cup with milk. Kitchen witch…please correct me if i am wrong…i have been before.