How were specific hentaigana selected for writing?


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?




So according to [this
article](https://www.jstage.jst.go.jp/article/johokanri/58/6/58_438/_html/-char/ja/)
(which is also cited in [this
section](https://ja.wikipedia.org/wiki/%E5%A4%89%E4%BD%93%E4%BB%AE%E5%90%8D#%E4%BD%BF%E3%81%84%E5%88%86%E3%81%91)
of Wikipedia), there were several factors that affected the choice of
hentaigana:

  1. Certain words, such as けふ, preferred certain kana.
  2. Certain kana were predominantly used at the beginning of a word. This can somewhat be compared to the [Arabic alphabet](https://en.wikipedia.org/wiki/Arabic_alphabet) where almost every character has initial, medial and final forms.
  3. Some kana were preferred when used as a particle. We still distinguish わ/は, を/お, へ/え, so this is understandable.
  4. When the same sound occurred consecutively, two distinct kana were employed to avoid repetition. Maybe this was aesthetically pleasing to people in the past.
  5. Sometimes, different hentaigana actually represented different sounds. In the old days when [濁点 was optional](https://japanese.stackexchange.com/a/100165/5010), 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.

