Th (digraph)

[1] The digraph ⟨th⟩ was first introduced in Latin to transliterate the letter theta ⟨Θ, θ⟩ in loans from Greek.

[2] ⟨th⟩ is used in academic transcription systems to represent letters in south and east Asian alphabets that have the value /tʰ/.

During late antiquity, the Greek phoneme represented by the letter ⟨θ⟩ mutated from an aspirated stop /tʰ/ to a dental fricative /θ/.

In the Norman dialect Jèrriais, the French phoneme /ʁ/ is realized as /ð/, and is spelled ⟨th⟩ under the influence of English.

This practice was then borrowed into German, French, Dutch and other languages, where ⟨th⟩ still appears in originally Greek words, but is pronounced /t/.

For example: Irish and Scottish Gaelic toil [tɛlʲ] 'will' → do thoil [də hɛlʲ] 'your will'.

Lenition in Gaelic lettering was traditionally denoted in handwriting using an overdot but typesetters lacked these pre-composed types and substituted a trailing ⟨h⟩.

It is also a consequence of their history: the digraph initially, in Old and Middle Irish, designated the phoneme /θ/, but later sound changes complicated and obscured the grapheme–sound correspondence, so that ⟨th⟩ is even found in some words like Scottish Gaelic piuthar 'sister' that never had a /θ/ to begin with.

This is an example of "inverted (historical) spelling": the model of words where the original interdental fricative had disappeared between vowels caused ⟨th⟩ to be reinterpreted as a marker of hiatus.