Historical kana usage is the system of kana (i.e., phonetic character) writing used in Japan before the post-war reforms. More specifically, it is the version of kana orthography standardized in the Meiji Period (since before that time kana usage was not standardized). It is, broadly speaking, based on the pronunciation of Japanese in the Heian Period, the time-frame in which Early Middle Japanese (on which the grammar of classical Japanese is based) was spoken. There are several differences between historical kana usage — which is also referred to as "old kana usage" ( "Kyū kana-zukai") — and the modern kana orthography, called "modern kana usage" ( "Gendai kana-zukai") or "new kana usage" ( "Shin kana-zukai"). Some of these differences apply primarily to Sino-Japanese readings of Chinese characters, while others apply primarily to native Japanese words, and still others apply equally to both groups of words.