The terms de jure and de facto are used instead of "in principle" and "in practice", respectively, when one is describing political or legal situations.
In a legal context, de jure is also translated as "concerning law". A practice may exist de facto, where for example the people obey a contract as though there were a law enforcing it yet there is no such law. A process known as "desuetude" may allow de facto practices to replace obsolete laws. On the other hand, practices may exist de jure and not be obeyed or observed by the people.