Th (digraph)

<th> for /th/

The most obvious use of the letter combination <th> is to represent the juxtaposition of the phonemes /t/ and /h/, as in English hothouse. However, this is not strictly a digraph, as it is representing a consonant cluster rather than a single phoneme.

<th> for /tʰ/

As a digraph in the strict sense, <th> was originally introduced in Latin, which had many Greek loan words, to transcribe the Greek letter Theta (<Θ>, <θ>), which at that time originally represented the aspirated stop /tʰ/.

Still today, <th> is used in academic transcription systems to represent letters in oriental alphabets which have the value /tʰ/. According to Royal Thai General System of Transcription, for example, <th> represents a series of Thai letters with the value /tʰ/.

<th> for /θ/

A sound shift in Greek in the last two centuries BC resulted in the letter <θ> coming to denote the voiceless dental fricative (IPA: /θ/). Thus by default, the Latin digraph also came to have this value in Greek loanwords.

This was then borrowed into the spelling system of some other languages which had the phoneme /θ/, including English. Initially, although Old English was written in the Latin alphabet, it used runic letter <Þ> (thorn) for this sound (although in the Northumbrian dialect the spelling of <th> was used for this very phoneme), but from Middle English times these were replaced by <th> by analogy with the way Latin represented the same sound of Greek.

Other languages using <th> to represent /θ/ include Albanian and Welsh, in both of which the digraph is considered a distinct letter, and is found between <t> and <u> in alphabetical order. Old High German used it before the final phase of the High German consonant shift.

<th> for /ð/

The use of the digraph <th> to represent a voiced sound is actually rather surprising, but in English it is used also for the voiced dental fricative /ð/. The reason for this is that in Old English the sounds [θ] and [ð] stood in an allophonic relationship to each other and therefore did not need to be distinguished in spelling. In much the same way, s is used for both [s] and [z].

From English, this usage was borrowed into the spelling of Jèrriais, the Norman dialect of Jersey, where /ð/ corresponds to a French /r/.

<th> for /t/

Because neither /tʰ/ nor /θ/ were native sounds in Latin, an original <θ> in Greek loanwords soon came to be pronounced in Latin with /t/. They continued to be spelled with <th> in deference to their etymology. This practice was then borrowed into German, French and other languages, where <th> still appears in Greek loan words, but is pronounced /t/. See German orthography. Interlingua also employs this pronunciation.

In early modern times, French, German and English all expanded this by analogy to words in which there was no etymological reason for it, but for the most part the modern spelling systems have eliminated this. A rare example of unetymological <th> in English is the name of the River Thames.

In English, <th> for /t/ can also occur in loan-words from French or German, such as Neanderthal.

<th> for /t̪/

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

<th> for /h/

In Irish and Scottish Gaelic <th> represents the lenition of /t/. In word-initial position this is pronounced /h/ in most cases. For example (in both languages), toil [tɛlʲ] 'will' → do thoil [də hɛlʲ] 'your will'.

This use of digraphs with <h> to indicate lenition is an entirely separate system from the other uses, which all derive from Latin. While it is possible that the presence of digraphs with <h> in Latin may have inspired the Celtic usage, their allocation to phonemes is based entirely on the internal logic of the Celtic languages.

<th> for /Ø/

The Irish and Scottish Gaelic lenited /t/ is silent in final positions. Scottish Gaelic sgith /ski:/ 'tired'. Exceptionally also in initial position (Scottish Gaelic thu /u:/ 'you').