Why is 「てふ」 pronounced 「ちょう」?


I recently purchased a copy of [One Hundred Leaves: A new annotated
translation of the Hyakunin
Isshu](https://leer.amazon.com.mx/kp/embed?asin=B0081UN2HY&preview=newtab&linkCode=kpe&ref_=cm_sw_r_kb_dp_B-
ZCAbGAEDDZF) to try and improve my Japanese reading level. In addition to the
English translation, each poem also includes the original Japanese and a
rōmaji pronunciation guide.

The [second
poem](https://en.wikipedia.org/wiki/Empress_Jit%C5%8D#Hyakunin_Isshu_poetry)
in the collection is by [Empress Jitō
(持統天皇)](https://en.wikipedia.org/wiki/Empress_Jit%C5%8D). In the book, the
Japanese text is given as (emphasis mine):

> 春過ぎて  
>  夏来にけらし  
>  白妙の  
>  衣ほす **てふ**  
>  天の香具山

The corresponding rōmaji for `てふ` is given as `chou`. I understand that
pronunciation can change over time, e.g. the particles `は`, `へ`, and `を`. But
`てふ ⟶ ちょう` is a less obvious change.

Can someone please explain why `てふ` is pronounced `ちょう`?

## Related info

I found [this page](http://www.ogurasansou.co.jp/site/hyakunin/002.html) which
I think is trying to tell me that `てふ` is a contracted form of `といふ`, i.e.
`と[言]{い}う`. Indeed, the [English version of the poem on
Wikipedia](https://en.wikipedia.org/wiki/Ogura_Hyakunin_Isshu#Poems)
translates the `てふ` as "So they say". But I still don't understand how this
relates to the pronunciation being `ちょう`.

I also looked for the answer on the Wikipedia page for [classical Japanese
language](https://en.wikipedia.org/wiki/Classical_Japanese_language), which
gives a bunch of examples of differences of pronunciation between modern and
classical Japanese, but I don't understand the explanations well enough to
determine if some rule or combination of rules listed on that page can explain
`てふ ⟶ ちょう` (or was it `ちょう ⟶ てふ`?).

Finally, the [kotobank.jp entry for
`ちょう〔てふ〕`](https://kotobank.jp/word/%E3%81%A1%E3%82%87%E3%81%86-568032#E3.83.87.E3.82.B8.E3.82.BF.E3.83.AB.E5.A4.A7.E8.BE.9E.E6.B3.89)
mentions something about an `[音変化]{おんへんか}` and seems to indicate an
intermediate step in the pronunciation change, i.e. `ちょう→ちゅう→とう`? Or maybe I
am misunderstanding what the `→ちゅう→とう` on that page is trying to tell me.




The best answer that I could find was [this
answer](https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q12152433092)
on Yahoo.jp 知恵袋, which goes:

「てふ」→(ハ行転呼)→「てう」

「てう」→(拗長音化)→「チョー(表記は「ちょう」)」

> 頭語以外の「はひふへほ」を原則「わいうえお」にする というのは「ハ行転呼」ですが、 これが起こった上で、さらに発音が変化して
> 「チョー(ちょう)」になっています。

「けふ(今日)」→「きょう」、

「きふ(急)」→「きゅう」、

なども、同じことが起きています。

* * *

[ハ行転呼](https://kotobank.jp/word/%E3%83%8F%E8%A1%8C%E8%BB%A2%E5%91%BC%E9%9F%B3-357187)

[拗長音](https://kotobank.jp/word/%E6%8B%97%E9%95%B7%E9%9F%B3-653233)




The best answer that I could find was [this
answer](https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q12152433092)
on Yahoo.jp 知恵袋, which goes:

「てふ」→(ハ行転呼)→「てう」

「てう」→(拗長音化)→「チョー(表記は「ちょう」)」

> 頭語以外の「はひふへほ」を原則「わいうえお」にする というのは「ハ行転呼」ですが、 これが起こった上で、さらに発音が変化して
> 「チョー(ちょう)」になっています。

「けふ(今日)」→「きょう」、

「きふ(急)」→「きゅう」、

なども、同じことが起きています。

* * *

[ハ行転呼](https://kotobank.jp/word/%E3%83%8F%E8%A1%8C%E8%BB%A2%E5%91%BC%E9%9F%B3-357187)

[拗長音](https://kotobank.jp/word/%E6%8B%97%E9%95%B7%E9%9F%B3-653233)

