The /ɑ/ vowel is usually spelled <ar> but we also find <al> in calf and <aa> in baas. The /ɒ/ vowel is spelled <o> followed by two consonants or one consonant in many monosyllables.
This contrast is between a long and a short vowel, but the tongue positions are very close. It is a recognised problem for Spanish and Swahili speakers, and probably causes confusion for many other groups of learners. In many dialects of American English the sounds are not distinguished, so those pairs where there is no <r> in the spelling would be homophones, as in the well-known example impassable/impossible.
Interesting pairs include:
ardour odder
armlet omelette
carton cotton
faster foster
larger lodger
The density value is 4.44%. The pairs make 98 semantic contrasts, giving a loading of 53.3%.
aft oft arcs ox ardour odder armlet omelette armlets omelettes baas boss balm bomb barb bob barbed bobbed barbs bobs barks box barn Bonn Barnet bonnet brass Bros calf cough Cali collie calmer comma Calne con can't cont card cod carded codded carding codding cards cods Carl col carp cop carped copped carping copping carps cops carped Copt cart cot carts cots cartage cottage carter cottar carters cottars carton cotton cartons cottons cast cost casts costs casting costing castings costings caste cost castes costs clerk clock clerked clocked clerking clocking clerks clocks craft croft crafts crofts daft doffed dark dock darker docker darling dolling darn don darned donned darning donning darns dons dart dot darted dotted darting dotting darts dots farce fosse farces fosses faster foster garb gob garbs gobs garble gobble garbled gobbled garbles gobbles garbling gobbling gargle goggle gargled goggled gargles goggles gargling goggling garner goner garners goners garner gonna Ghana goner Ghana gonna glass gloss glassed glossed glasses glosses glassing glossing glassy glossy glassier glossier glassiest glossiest Graz grots guard God guards gods hard hod hark hock harked hocked harking hocking harks hocks Harlow holloa harp hop harped hopped harping hopping harps hops harper hopper harpers hoppers Hartley hotly hasp Hosp hart hot heart hot hearted hotted hearts hots impassable impossible karma comma karmas commas khaki cocky khans cons lakh loch lakhs lochs lards Lodz larger lodger Largs logs lark lock larked locked larking locking larks locks last lost laughed loft Lens lance Mali Molly mark mock marked mocked marking mocking marks mocks marker mocker markers mockers mark-up mock-up mark-ups mock-ups marl moll marred Mod mask mosque masks mosques nark knock narked knocked narking knocking narks knocks palm pom palms poms palmy pommy park pock parked pocked parks pocks parley poly parleys polys part pot parted potted parting potting parts pots party potty parties potties passable possible passably possibly pasture posture pastured postured pastures postures pasturing posturing repast riposte repasts ripostes R's oz sahib sob sahibs sobs sari sorry scarf scoff scarfs scoffs shard shod shark shock sharks shocks sharp shop sharps shops sharper shopper sharpers shoppers stark stock tardy toddy tart tot tarted totted tarting totting tarts tots tartar totter tartars totters yarn yon
John Higgins, Shaftesbury, June 2016