The /f/ sound is spelled with <f> or <ff>, and exceptionally <gh> in laugh. The /Ɵ/ sound is spelled <th> throughout.
The contrast is between a labio-dental fricative and a dental and alveolar fricative, both voiceless. The sounds are very similar and can be a source of misunderstanding even among native speakers. Novelists often replace <th> with <f> in dialogue to suggest working-class speech: "That's what I fink" or "Fings ain't wot vey used to be." David Lodge exploits the contrast in the title of his novel about the problems of increasing deafness in old age, Deaf Sentence, with its punning allusion to the familiar collocation "Death sentence", and which he admits poses a huge problem for translators.
The mean density value is 1.2%, but the true value is probably lower since a number of the <th> words in the list are unusual or obsolescent: Cathay, thane, thews, thole and withe. This suggests some support for the O'Connor conjecture that language is self-repairing. The list makes 42 semantic distinctions, a loading of 69%.
beef Beith café Cathay caff Cath deaf death fain thane faugh thaw faun thorn fauns thorns feign thane feigns thanes feoff thief fie thigh firm therm firms therms first thirst firsts thirsts fin thin fins thins Finn thin Finns thins foal thole foals tholes for thaw ford thawed fore thaw fort thought forts thoughts fought thought four thaw fours thaws fro throe froze throes Fred thread fresh thresh fret threat frets threats free three freeze threes fresher thresher freshers threshers frill thrill frilled thrilled frills thrills fug thug fugs thugs fuse thews half hearth infuse enthuse infused enthused infusing enthusing infuses enthuses leafy Lethe loaf loath laugh lath oaf oath oafs oaths roof Ruth sheaf sheath whiff with whiff withe whiffs withes
John Higgins, Shaftesbury, October 2010.