Th (digraph)
Th is a digraph in the Latin script. It was originally introduced into Latin to transliterate Greek loan words. In modern languages that use the Latin alphabet, it represents a number of different sounds. It is the most common digraph in order of frequency in the English language. Cluster /t.h/

The most logical use of  is to represent a consonant cluster of the phonemes  and , as in English knighthood. This is not a digraph, since a digraph is a pair of letters representing a single phoneme or a sequence of phonemes that does not correspond to the normal values of the separate characters. Aspirated stop /tʰ/

The digraph  was first introduced in Latin to transliterate the letter theta  in loans from Greek. Theta was pronounced as an aspirated stop  in Classical and early Koine Greek. is used in academic transcription systems to represent letters in south and east Asian alphabets that have the value . According to the Royal Thai General System of Transcription, for example,  represents a series of Thai letters with the value . is also used to transcribe the phoneme  in Southern Bantu languages, such as Zulu and Tswana. Voiceless fricative /θ/

During late antiquity, the Greek phoneme represented by the letter  mutated from an aspirated stop  to a dental fricative . This mutation affected the pronunciation of , which began to be used to represent the phoneme  in some of the languages that had it. One of the earliest languages to use the digraph this way was Old High German, before the final phase of the High German consonant shift, in which  and  came to be pronounced . In early Old English of the 7th and 8th centuries, the digraph  was used until the Old English Latin alphabet adapted the runic letter  (thorn), as well as  (eth;  in Old English), a modified version of the Latin letter , to represent this sound. Later, the digraph reappeared, gradually superseding these letters in Middle English. In modern English, an example of the  digraph pronounced as  is the one in tooth. In Old and Middle Irish,  was used for  as well, but the sound eventually changed into  (see below). Other languages that use  for  include Albanian and Welsh, both of which treat it as a distinct letter and alphabetize it between  and . Voiced fricative /ð/

English also uses  to represent the voiced dental fricative , as in father. This unusual extension of the digraph to represent a voiced sound is caused by the fact that, in Old English, the sounds  and  stood in allophonic relationship to each other and so did not need to be rigorously distinguished in spelling. The letters  and  were used indiscriminately for both sounds, and when these were replaced by  in the 15th century, it was likewise used for both sounds. (For the same reason,  is used in English for both  and .) In the Norman dialect Jèrriais, the French phoneme  is realized as , and is spelled  under the influence of English. Voiceless retroflex stop /ʈ/

In the Latin alphabet for the Javanese language,  is used to transcribe the phoneme voiceless retroflex stop , which is written as  in the native Javanese script. Alveolar stop /t/

Because neither  nor  were native phonemes in Latin, the Greek sound represented by  came to be pronounced . The spelling retained the digraph for etymological reasons. This practice was then borrowed into German, French, Dutch and other languages, where  still appears in originally Greek words, but is pronounced . See German orthography. Interlingua also employs this pronunciation. In early modern times, French, German and English all expanded this by analogy to words for which there is no etymological reason, but for the most part the modern spelling systems have eliminated this. Examples of unetymological  in English are the name of the River Thames from Middle English  and the name Anthony (though the  is often pronounced  under the influence of the spelling) from Latin . In English,  for  can also occur in loan-words from French or German, such as Neanderthal. The English name Thomas has initial  because it was loaned from Norman. Dental stop /t̪/

In the transcription of Australian Aboriginal languages  represents a dental stop, . /h/

In Irish and Scottish Gaelic,  represents the lenition of . In most cases word-initially, it is pronounced . For example: Irish and Scottish Gaelic   'will' →   'your will'. This use of digraphs with  to indicate lenition is distinct from the other uses which derive from Latin. While it is true that the presence of digraphs with  in Latin inspired the Goidelic usage, their allocation to phonemes is based entirely on the internal logic of the Goidelic languages. Lenition in Gaelic lettering was traditionally denoted in handwriting using an overdot but typesetters lacked these pre-composed types and substituted a trailing . It is also a consequence of their history: the digraph initially, in Old and Middle Irish, designated the phoneme , but later sound changes complicated and obscured the grapheme–sound correspondence, so that  is even found in some words like Scottish Gaelic  'sister' that never had a  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  to be reinterpreted as a marker of hiatus. Ø

The Irish and Scottish Gaelic lenited  is silent in final position, as in Scottish Gaelic   'tired'. And, rarely, it is silent in initial position, as in Scottish Gaelic   'you'. In English, the  in asthma and clothes is often silent. ᵺ

 is used for phonetic notation in some dictionaries. See also

Pronunciation of English ⟨th⟩

Footnotes

English th
Latin-script digraphs
Graphemes