The usage and meaning of a monarch's specific title has historically been defined by tradition, law and diplomatic considerations.
Note that some titles borne by monarchs have several meanings and may not exclusively designate a monarch. A Prince may be a person of royal blood (some languages uphold this distinction, see Fürst). A Duke may belong to a peerage and hold a dukedom (title) but no duchy (territory). In Imperial Russia, a Grand Duke was a son or patrilineal grandson of the Tsar or Tsarina. Holders of titles in these alternative meanings did not enjoy the same status as monarchs of the same title.
Within the Holy Roman Empire, there were numerous titles used by noblemen whose authority within their territory sometimes approached sovereignty, even though they acknowledged the Holy Roman Emperor as suzerain; Elector, Grand Duke, Margrave, Landgrave and Count Palatine, as well as kings, dukes, princes and "princely counts" (Gefürstete Grafen). A ruler with a title below emperor or king might still be regarded as a monarch, outranking a nobleman of the same ostensible title (e.g., Antoine, Duke of Lorraine, a reigning sovereign, and his younger brother, Claude, Duke of Guise, a nobleman in the peerage of France).