"Chūgū" was a term which evolved during the Heian period; and it came to be understood as the title of the empress. For a time, "chūgū" replaced "kōgō"; and then the titles became interchangeable.
t̠ʃ uː ɡ uː WORD_BOUNDARY w ʌ z ʌ WORD_BOUNDARY t ɜː m WORD_BOUNDARY w ɪ t̠ʃ WORD_BOUNDARY ɪ v ɑ l v d WORD_BOUNDARY d ʊ ɹ ɹ ɪ ŋ WORD_BOUNDARY ð ə WORD_BOUNDARY h aɪ ə n WORD_BOUNDARY p iə ɹ ɪ ə d WORD_BOUNDARY æ n d WORD_BOUNDARY ɪ t WORD_BOUNDARY k eɪ m WORD_BOUNDARY t ə b i WORD_BOUNDARY ʌ n d ə ɹ s t ʊ d WORD_BOUNDARY æ z WORD_BOUNDARY ð ə WORD_BOUNDARY t aɪ t ə l WORD_BOUNDARY ʌ v ð ɪ WORD_BOUNDARY ɛ m p ɹ ɛ s WORD_BOUNDARY f ə ɹ ɹ ə WORD_BOUNDARY t aɪ m WORD_BOUNDARY t̠ʃ uː ɡ uː WORD_BOUNDARY ɹ ɪ p l eɪ s t WORD_BOUNDARY k oʊ ɡ oʊ WORD_BOUNDARY æ n d WORD_BOUNDARY ð ɛ n WORD_BOUNDARY ð ə WORD_BOUNDARY t aɪ t ə l z WORD_BOUNDARY b ɪ k eɪ m WORD_BOUNDARY ɪ n t ə ɹ t̠ʃ eɪ n d̠ʒ ə b ə l WORD_BOUNDARY