GH
GH, gh is a digraph (a two-letter grapheme) used with various different values in a number of languages using the Latin alphabet, especially in English, Irish Gaelic, Scottish Gaelic, Cornish, Italian, Romanian, Friulian and Corsican.
Use in English
Except at the beginning of a word, where it sounds like a hard g, as in ghôst, GH is a notorious digraph in English. It occurs in nîght and cóugh, for example, pronounced *nîte and *cóff (the accents show pronunciation: see English phonemes). In such cases, GH is the sorry relic of a sound (IPA χ) no longer pronounced except in exclamations of disgust, úgh! yeùgh!, the sound of Scottish ch in lóch (which in Ireland is indeed spelt lóugh) - or mutated into the sound of [f] and 'ph'.
It is pronounced [f] in: tróugh, cóugh, Góugh, enoúgh, toúgh, roúgh, sloúgh skin (cf. slòugh swamp, and the English town Slòugh, both *slòu).
More often it is silent as in slòugh swamp - with quite a variety of preceding vowel sounds and spellings: ŏught, sŏught, bŏught, cåught, nåughty, Våughan, Våughn, dôugh, èight, nèigh, wèigh, slèigh ride (= slây kill), wèight heavy (= wâit time), frèight, heîght, bòugh, throûgh, thôugh, Búrrôughs, sîght, nîght, nîgh, and ough is even sometimes a schwa [ə] as in BrE bòrough, Scàrborough and thòrough, which in AmE are bòrôugh, Scàrborôugh, and thòrôugh, rhyming with fúrrôw. British English pronounces fürlôugh this way too.
gh uniquely sounds like [p] in híccoúgh (a variant spelling of híccup). In other words the digraph merely represents a hard g, whether Germanic, as in ghôst, ghoûl, ghāstly, or Italian, as in spaghéttì; and h serves to distinguish dínghy boat (which can have hard g or silent g, but always the ng sound) from díngy dirty (soft g: *dínjy).