The spelling of /t/ is usually <t>, <te> or <tt>, but we also often see <ght> in naught and <th> in Thai. The /Ɵ/ sound is <th> throughout.
The contrast is between a plosive and a fricative, both voiceless and close together in the mouth. It is a problem for many learners, including Dutch and French.
An interesting mis-hearing arising from this contrast occurred in BBC news broadcasts on 26 December 2011, when it was announced that "Following heart surgery, Prince Philip will spend a fourth night in hospital." This was often heard as "a fortnight in hospital", which would have been far more serious.
One potentially embarrassing pair is turd/third.
Interesting pairs include:
The mean density value is 0.8%. The list makes 91 semantic distinctions, a loading of 71%.
Bart bath beat Beith beet Beith Bert berth Bert birth boat both cat Cath clot cloth clots cloths dirt dearth debt death debts deaths eater aether eater ether fate faith fates faiths fête faith fêtes faiths fort forth fort fourth forts fourths fought forth fought fourth garret Gareth girt girth got Goth groat growth groats growths hart hearth harts hearths heart hearth hearts hearths hat hath heat heath heats heaths kit kith mats maths meat Meath Metz meths mitt myth mitts myths naught north nutting nothing oat oath oats oaths part path pert Perth pit pith pity pithy plating plaything quote quoth rate wraith rates wraiths rot wrath rote wroth root ruth set saith sheet sheath suit sooth tank thank tanked thanked tanking thanking tanks thanks taught thought taut thought tawny thorny tawse thaws team theme teams themes teat teeth tent tenth tenths tents term therm terms therms Thai thigh Thais thighs tie thigh ties thighs tor thaw tors thaws tore thaw tic thick tick thick ticker thicker ticket thicket tickets thickets tinker thinker tinkers thinkers turd third turds thirds toll thole tolls tholes tongs thongs torn thorn tort thought torts thoughts trawl thrall trawls thralls trash thrash tread thread treading threading treads threads tree three trees threes true threw true through trice thrice trill thrill trilled thrilled trilling thrilling trills thrills trove throve trussed thrust trust thrust trusting thrusting trusts thrusts tug thug tugs thugs tilt tilth tilts tilths toot tooth welt wealth wert worth whit withe whit with wit with wit withe wits withes
John Higgins, Shaftesbury, December 2011.