I've been learning how to read older documents and trying to familiarize myself with hentaigana, but as I've started this, I've began to wonder if there is any rhyme or reason to the choices writers make when they select one hentaigana over another. Is it just a matter of the current trend? Is it a matter of style, flow, and aesthetics (especially perhaps in documents written in kuzushi-ji?), or is there something else going on here. If one used 可 for "ka" once, would it be odd for the writer to use "加" later on? What are the circumstances and rules that dictate these sorts of things?
- Certain words, such as けふ, preferred certain kana.
- Certain kana were predominantly used at the beginning of a word. This can somewhat be compared to the Arabic alphabet where almost every character has initial, medial and final forms.
- Some kana were preferred when used as a particle. We still distinguish わ/は, を/お, へ/え, so this is understandable.
- When the same sound occurred consecutively, two distinct kana were employed to avoid repetition. Maybe this was aesthetically pleasing to people in the past.
- Sometimes, different hentaigana actually represented different sounds. In the old days when 濁点 was optional, the ち and ぢ sounds were sometimes denoted using different hentaigana.
Note that these rules have varied significantly over time. Since Man'yōgana, there seems to have been a trend to prioritize diversity over standardization when it came to hiragana. The unification happened only after the Meiji era. Contrastively, for katakana, there seems to have been an almost 1:1 correspondence established relatively early on.