The /Ɵ/ sound is spelled with <th>. The /z/ sound is spelled with <s> or <se> or <z> in zinc.
The /Ɵ/ is a voiceless dental fricatives. /z/ is a voiced sibilant, fairly close in the mouth. The contrast is not a great problem, and there are in any case rather few pairs. A good many are accounted for by the pairing of ordinal numbers and plural forms for the numerals 4, 7, 9, 10, 11, 13, 14, 15, 16, 17, 18, 19 and 100. I have left in the archaic doth and saith, though they hardly figure in modern speech outside church contexts.
The mean density value is 0.7%. The list makes 71 semantic distinctions, a loading of 97%.
bath baas bath bars Beith B's Beith bees berth burrs both beaux both bows death Des doth does earth errs eighteenth eighteens eleventh elevens faith phase fifteenth fifteens filth fills firth firs firth furs firth furze fort fought fourteenth fourteens fourth fours growth grows hath has health hells heath he's hundredth hundreds Keith keys Keuth quays loth lows millionth millions mouth mows myth Ms 'neath knees nineteenth nineteens ninth nines north gnaws oath O's oath owes path pars path parse Perth purrs ruth rues ruth ruse saith says seventeenth seventeens seventh sevens sleuth slews sloth sloes sloth slows sooth sous south sows strewth strews teeth T's teeth teas teeth tease tenth tens thing zing things zings think zinc thinks zincs thirteenth thirteens tilth tills tooth twos trillionth trillions truth trews warmth warms wealth wells withe whiz worth whirrs wraith raise wraith rays youth U's youth use
John Higgins, Shaftesbury, November 2010.