The term de facto standard is used in contrast with obligatory standards (also known as "de jure standards"); or to express the dominant voluntary standard, when there is more than one standard available for the same use.
In social sciences, a voluntary standard that is also a de facto standard is a typical solution to a coordination problem. The choice of a de facto standard tends to be stable in situations in which all parties can realize mutual gains, but only by making mutually consistent decisions. In contrast, an enforced "de jure standard" is a solution to the prisoner's problem.
A selection of well-known and illustrative examples of de facto and de jure standards are:
Examples of long-time de facto but never de jure standards (for computer file formats):