"Historical kana usage", a prescriptive method of using kana to write Japanese. Devised in the Edo period by Keichū (契沖) and taught in schools until 1946, it aims to be etymologically faithful and reflects earlier stages of the language (Heian period and earlier).

historic, historical / already-happened, past / former, old, traditional

(linguistics) kana usage (orthography) / (linguistics) how letters or characters are read or pronounced ("used") / a method of writing (or "using") letters or characters

he / distal demonstrative, something far off removed from both speaker and listener: that / boyfriend

distal demonstrative, something far off removed from both speaker and listener: that, yon / particularly, male personal third person pronoun: he / by extension from he: boyfriend

he; boyfriend

history / the study of history

experimental study

a piece of research, a study, investigation

to research, to study