When it comes to civil and criminal cases, the Bundesgerichtshof is at the top of the hierarchy of courts. The other branches of the German judicial system each have their own appellate systems, each topped by a high court; these are the Bundessozialgericht for matters of social security, the Bundesarbeitsgericht for employment and labour, the Bundesfinanzhof for taxation and financial issues, and the Bundesverwaltungsgericht for administrative law. The so-called Gemeinsamer Senat der Obersten Gerichtshöfe (Joint Senate of the Supreme Courts) is not a supreme court in itself, but an ad-hoc body that is convened in only when one supreme court intends to diverge from another supreme court's legal opinion or when a certain case exceeds the authority of one court. As the courts have well-defined areas of responsibility, situations like these are rather rare and so, the Joint Senate gathers very infrequently, and only to consider matters which are mostly definitory.