Th is a digraph in the Latin script. It was originally introduced into Latin to transliterate Greekloan 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 /t/ and /h/, 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.
During late antiquity, the Greek phoneme represented by the letter mutated from an aspirated stop to a 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. The Old English Latin alphabet adapted the runic letter , as well as , a modified version of the Latin letter, to represent this sound, but the digraph gradually superseded these letters in Middle English. However, in early Old English of the 7th and 8th centuries, the runic letters were initially not used yet and the digraph used in its place. 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 . Other languages that use for include Albanian and Welsh, both of which treat it as a distinct letter and alphabetize it between T| and U|.
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 /θ/ and /ð/ 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. In the Norman dialect Jèrriais, the French phoneme is realized as, and is spelled under the influence of English.
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 Temese and the name Anthony from Latin Antonius. 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.
In Irish and Scottish Gaelic, represents the lenition of. In most cases word-initially, it is pronounced. For example: Irish and Scottish Gaelic toil 'will' → do thoil '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. 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 piuthar "sister" that never had a to begin with. This is an example of "inverted 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 /t/ is silent in final position, as in Scottish Gaelic sgith "tired". And, rarely, it is silent in initial position, as in Scottish Gaelic thu "you". In English the in "asthma" and "clothes" is often silent.
