仮名遣い. usage of kana, は vs. わ, 書く vs. かく etc.
When to use kana and which kana to use? The two kana syllabaries are fairly phonetic scripts, but there are some unique or unusual spellings and uses. Small kana, lengthened vowels, kana for imported words, small-tsu, old kana (ゑびす), kana-only instead of kanji etc.