昨日の記事「#4108. boy, join, loiter --- 外来の2重母音の周辺的性格」 ([2020-07-26-1]) で触れたように，2重母音 /ɔɪ/ は歴史的に注目すべき背景を有している．この話題について，もう少し掘り下げてみたい．
　現代英語の /ɔɪ/ に連なる中英語での実現形は， [oi] あるいは [ui] だった．これらが結果的に現代の /ɔɪ/ へ一本化していったわけだが，特に中英語 [ui] についてはスペリングとの関係でも興味深い歴史的背景がある．Minkova (269--70) を引用しよう．

The original [ui] has an unusual history in that, like the more recent history of /h-/ . . . , it is one of the rare well-documented instances of a sound change in progress inhibited and partially reversed by the influence of spelling. Many of the words with PDE <oi> --- loin, boil, coy, oil, join, point, choice, poison --- had variant pronunciations with [oi] and [ui], the latter commonly from Anglo-Norman. The 'normal' development of [ui], involving lowering and centralisation of the first element of the diphthong . . . , was towards [əi], which in the seventeenth and into the eighteenth century was also a possible realisation of historical [iː]. That the two etymologically distinct entities were treated as identical is shown by rhymes like loin: line, boil: bile, point: pint as late as the second half of the eighteenth century. In spelling the [ui] > [əi] words alternated between <ui> and <oi>. Eventually the centralised pronunciation [əi] for historical [ui] was abandoned in favour of [ɔi], no doubt supported by the spelling and pronunciation of the majority of the loanwords in that group.

　中英語 [ui] は通常の音変化のルートに乗っていれば，近代では [əi] となっていたはずであり，実際にそうなっていたのだが，後期近代以降，綴字が <oi> として保たれていたこと，そして <oi> ≡ [ɔi] が多くの単語において確立していたことから，ある種の人為的な [əi] > [ɔi] という変化が生み出されたという．要するに，綴字発音 (spelling_pronunciation) の例である．

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