An invert-emulsion mud, or an emulsion whose continuous phase is oil. In the past, the term referred to an oil mud containing less than about 5 vol. % water. This definition, at the time, distinguished mud with less than 5 vol. % water from invert-emulsion oil muds, which had more than 5 vol. % water. Today, this distinction is not practical because most commercial oil muds can be formulated with more or less than 5 vol. % water using essentially the same types of products.