Soon after Japan established diplomatic relations with the West at the end of Edo Period, the Japanese government misunderstood that the relationship between the emperor (Japanese emperor) and king of the West was an unequal relationship like that of Japan and China, and discriminated sovereign's messages to imperial states from those to royal states, causing confusions.
なお、幕末以後日本が西洋と国交を開いた直後には、西洋の皇帝（＝天皇）と王の関係を日本や中国と同様の非対等な関係と誤解をして、帝制の国家と王制の国家あての国書に格差を付けてしまった為に、紛糾したと言われている。