A Queen of the Romans also became Holy Roman Empress if her husband was crowned Holy Roman Emperor, in the Middle Ages usually by the Pope in Rome during an Italienzug. Most elected Kings of the Romans did, but some never made it that far, and thus their wives only ever achieved the status of Queen of the Romans.
A woman who married a King of the Romans could rise to become Holy Roman Empress if her husband was crowned Emperor by the Pope in Rome during an Italian military campaign. However, not all Kings of the Romans achieved this status, leaving their wives with only the title of Queen of the Romans.