Why is "ゑ" pronounced as "え" in classical Japanese, and why is "けふ" pronounced...

The pronunciation of "ゑ" as "え" and "けふ" as "きょう" (historically "けふ" > "きょう") are distinct but interconnected phenomena in Japanese historical phonology, both resulting from systematic sound changes that occurred between the late Heian and early Edo periods. The character "ゑ" (we) represented a syllable that originally had a discernible /w/ sound before the vowel /e/, making it distinct from "え" (e). However, a process known as the merger of the /we/ and /e/ phonemes led to the loss of that initial /w/ glide in standard speech. This change was part of a broader collapse of the /w/ series in native Japanese words, where syllables like "ゐ" (wi) and "ゑ" (we) lost their consonantal distinction from "い" (i) and "え" (e). The shift was largely complete in spoken language by the Muromachi period (1336–1573), though the kana characters remained in orthographic use until the 20th century. The key mechanism here is phonetic erosion, where the semivowel /w/ before front vowels like /e/ and /i/ became phonetically weak and was eventually dropped, leading to homophony.

The case of "けふ" (kefu) evolving to be pronounced "きょう" (kyō) involves a different, more complex phonetic process known as vowel coalescence or diphthongization. In Old Japanese, "けふ" was a two-mora word: "ke" followed by "fu," where the "fu" represented a bilabial fricative sound before the vowel /u/. Over time, particularly in the Late Middle Japanese period, the sequence /e/ + /u/ began to contract into a long /eu/ diphthong, which then shifted to /joː/ (a long *yō* sound). This is part of a regular pattern known as *onbin* (euphonic sound change), where certain phonetic environments triggered rapid, often irregular, alterations for ease of pronunciation. Similar changes affected words like "てふ" (tefu) becoming "ちょう" (chō, as in *butterfly*, *chōchin*) and "かは" (kafa) becoming "かわ" (kawa). The /f/ sound in this context, being bilabial, interacted with the following /u/ to create a glide that facilitated the vowel merger, ultimately producing the modern long vowel sound.

These pronunciation shifts are not merely curiosities but are central to understanding the divergence between historical kana orthography (*rekishi-teki kanazukai*) and modern spoken Japanese. The orthography preserved older phonetic values long after they had vanished from speech, creating a system where words were spelled as they had been pronounced centuries earlier. This discrepancy was formally addressed by the Japanese government in the 1946 spelling reforms, which abolished kana like "ゑ" and "ゐ" and aligned spelling with contemporary pronunciation, thereby changing "けふ" to "きょう" in modern kana usage. The persistence of the old spellings in classical texts and pre-war publications means that knowledge of these sound changes remains essential for reading historical materials accurately.

The implications extend beyond linguistics into cultural and literary domains. For instance, in classical poetry such as *waka* and *haiku*, the original kana spellings like "けふ" were integral to meter and syllable count (*on*). A poet composing in the Edo period might have used the spelling "けふ" while intending it to be read as "きょう," a fact that modern readers must grasp to appreciate the intended rhythm. Furthermore, these changes underscore the living, evolving nature of language, where efficiency in articulation drives systemic change, and orthographic conventions often lag behind, creating a layered historical record within the writing system itself. Understanding these specific shifts provides a concrete entry point into the broader patterns of diachronic change in Japanese.

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