The spelling of /p/ is either <p> or <pp>. The /ð/ sound occurs initially only in function words where it is spelled <th>. This limits the number of contrasts with initial /p/, though this is extended by including contracted forms such as they'll and there's. When /ð/ occurs medially or finally it is usually spelled <the> or <th> before <i>. I have included inflected forms where the contrast is final, such as reaped/wreathed and reaps/wreathes, even though these are not striuctly minimal.
The contrast is between a voiceless stop and a voiced fricative, well separated in the mouth so that confusions are unlikely, even though /ð/ is a difficult sound for many learners.
Interesting pairs include
dipper dither
leper leather
pence thence
slipper slither
upper other
zipper zither
The mean density value is 1%. The list makes 52 semantic distinctions, a loading of 71%.
dipper dither dippers dithers leper leather lepers leathers lope loathe loping loathing lopes loathes P thee P's these paid they'd pail they'll pair their pairs theirs pair there pair they're pairs there's pale they'll pan than pare their pares theirs pare there pare they're pares there's pat that pats that's pattered that'd pave they've pay they pea thee pear their pears theirs pear there pear they're pears there's peas these pease these pee thee pees these pen then pence thence pi thy pie thy pine thine piss this poppas pothers pose those P's these pus thus reap wreathe reaping wreathing reaped wreathed reaps wreathes ripe writhe seep seethe seeping seething seeped seethed seeps seethes sheep sheathe slipper slither slippered slithered slippers slithers slippery slithery soup soothe souping soothing soups soothes type tithe types tithes upper other uppers others whip with whippy withy zipper zither zippers zithers
John Higgins, Shaftesbury, January 2010.