The /f/ sound is spelled with <f> or <ff>, and exceptionally <gh> in cough and rough. The /ʃ/ sound is spelled <sh> or occasionally <sch>.
The contrast is between a labio-dental fricative and a palato-alveolar fricative, both voiceless. It is not usually a problem.
Taboo word pairs include fag/shag, fat/shat, fit/shit and fuck/shuck.
Interesting pairs include:
The mean density value is 2%. The list makes 82 semantic distinctions, a loading of 56%.
afore ashore bluff blush bluffed blushed bluffing blushing café cachet cafés cachets caff cache coffees coshes cough cosh coughed coshed coughing coshing fa shah fact shacked fad shad fade shade faded shaded fades shades fading shading fag shag fagged shagged fagging shagging fags shags fail shale fair share fairs shares fake shake fakes shakes faking shaking fakir shakier fall shawl falls shawls fallow shallow fame shame famed shamed famous Seamus far shah fare share fared shared fares shares faring sharing fat shat fatter shatter faugh shore faun Sean faun shorn favour shaver favours shavers fawn Sean fawn shorn fear shear feared sheared fearing shearing fears shears fed shed fee she fell shell felled shelled felling shelling fells shells ferry sherry ferries sherries fie shy field shield fielded shielded fielding shielding fields shields fifty shifty filling shilling fillings shillings fine shine fines shines fining shining fin shin fins shins Finn shin Finns shins fire shire fires shires fish shish fist schist fists schists fit shit fitted shitted fluff flush fluffed flushed fluffing flushing foal shoal foaled shoaled foaling shoaling foals shoals foe show foes shows folder shoulder folders shoulders food shooed fop shop fops shops for shore fore shore four shore fours shores ford shored fox shocks frank shrank freak shriek freaked shrieked freaking shrieking freaks shrieks Fred shred frill shrill frilled shrilled frills shrills frilly shrilly frivol shrivel frivolled shrivelled frivolling shrivelling frivols shrivels fuck shuck fucked shucked fucking shucking fucks shucks fun shun fund shunned gaff gash half harsh huff hush huffed hushed huffing hushing leaf leash luff lush muff mush phase chaise phases chaises phone shown phut shut quaff quash quaffed quashed quaffing quashing rough rush roughed rushed roughing rushing slough slush swift swished toff tosh toffees toshes waft washed whiff wish
John Higgins, Shaftesbury, October 2010.