Those <gh>'s are all that is left of two sounds that were common in Old English but have
disappeared from our language. They were velar and palatal fricatives--that is, fricative sounds
pronounced far back in the mouth, much like the sounds in the Scottish pronunciation of loch and
the German pronunciation of Bach. In Old English they were most often spelled <g> or <h>; in
Middle English they were usually spelled <gh>. In some pronunciations of a few quite rare words
<gh> still spells a velar fricative: curraugh, haugh, kiaugh, lough, quaigh, yogh.
But over time most of these Old English fricatives changed and eventually disappeared. As they
did so, they affected the vowels preceding them, lengthening some, turning others into
diphthongs. As a result today the <gh>'s that descend from Old English usually follow a long <i>
or another long vowel or diphthong spelled <ai>, <au>, <ei>, or <ou>:
bight, bright, dight, fight, flight, fright, high, hight, knight, light, might, nigh, night, plight,
right, sigh, sight, slight, thigh, tight, wight, wright;
eight, freight, neighbor, sleigh, weigh, weight;
dough, though, borough, thorough,
slough "bog", through;
bough, doughty, plough.
In a few cases the <gh> follows short [o] or [ spelled
<au> or <ou>:
aught, caught, daughter, distraught, fraught, haughty, naught, naughty, slaughter, taught;
bought, brought, fought, nought, ought, sought, thought, wrought.
And in a few cases <gh> spells [f] after <au> spelling [a] or <ou> spelling [u]:
draught, laugh, laughter;
clough, enough, rough, slough "discard", sough, tough.
A few words have been respelled with <gh> via analogy with the <gh> that descends from the
Old English fricatives: blight, chough, delight, distraught, furlough, haughty, inveigh, sleigh,
Other than the native burgh, none of the <gh> spellings of [g] descend from the Old English
fricatives. Ghost is probably a Flemish-influenced respelling of earlier gost by the first English
printer William Caxton, and aghast and ghastly are probably respellings via analogy with ghost.
The remainder are foreign adoptions — afghan, agha, barghest, carragheen, dinghy, ghat,
ghetto, ghoul, ogham, prioghi, sorghum, spaghetti, yoghurt, etc.
And finally, in a few words <gh> is the product of the concatenation of two bases, as in bighead
big+head: foghorn, leghorn, pigheaded, etc. (AES, 11.2.2, 126.96.36.199, 15.2.3, 188.8.131.52, and