Kaiser and Rex

Kaiser noun - The title of the Holy Roman Emperors or the emperors of Austria or of Germany until 1918.

Rex and kaiser are semantically related In some cases you can use "Rex" instead a noun "Kaiser".

Rex noun - A male sovereign; ruler of a kingdom.

Kaiser and rex are semantically related in ruler topic. Sometimes you can use "Kaiser" instead a noun "Rex", if it concerns topics such as historically.