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Gh is a digraph found in many languages.
In Latin-based orthographies 
In English, ⟨gh⟩ historically represented [x] (the voiceless velar fricative, as in the Scottish Gaelic word Loch). In modern English, ⟨gh⟩ is almost always either silent or pronounced /f/ (see ough). It is thought that before disappearing, the sound became partially or completely voiced to [ɣx] or [ɣ], which would explain the new spelling - Old English used a simple ⟨h⟩ - and the diphthongization of any preceding vowel.
When gh occurs at the beginning of a word in English, it is pronounced /ɡ/ as in "ghost", "ghastly", "ghoul", "ghetto", "ghee" etc. In this context, it does not derive from a former /x/.
American Literary Braille has a dedicated cell pattern for the digraph ⟨gh⟩ (dots 126, ⠣).
In Esperanto orthography, ⟨gh⟩ can be used when printers lack type for ⟨ĝ⟩.
In Irish, ⟨gh⟩ represents /ɣ/ (the voiced velar fricative) and /j/ (the voiced palatal approximant). Word-initially it represents the lenition of ⟨g⟩, for example mo ghiall [mə jiəl̪ˠ] "my jaw" (cf. giall [ɟiəl̪ˠ] "jaw").
The Maltese language has a related digraph, ⟨għ⟩. It is considered a single letter, called għajn (the same word for eye and spring, named for the corresponding Arabic letter 'ayin). It is usually silent, but it is necessary to be included because it changes the pronunciation of neighbouring letters, usually lengthening the succeeding vowels. At the end of a word (when not substituted by an apostrophe), it is pronounced [ħ]. Its function is thus not unlike modern English gh, except that the English version comes after vowels rather than before like Maltese għ (għajn would come out something like aighn if spelled as in English).
In Canadian Tlingit ⟨gh⟩ represents /q/, which in Alaska is written ⟨ǥ⟩.
In romanization 
In the romanization of various languages, ⟨gh⟩ usually represents the voiced velar fricative (/ɣ/). Like ⟨kh⟩ /x/, ⟨gh⟩ may also be pharyngealized, as in several Caucasian and Native American languages. In transcriptions of Indo-Aryan languages such as Sanskrit and Hindi, as well as their ancestor, Proto-Indo-European, ⟨gh⟩ represents a voiced velar aspirated plosive /ɡʱ/ (often referred to as a breathy or murmured voiced velar plosive)