The /əʊ/ diphthong is spelled in many ways including <oCe>, <oe>, <oa> and <ough> in though. The /aʊ/ diphthong is spelled <ou> or <ow>, and <au> in the loan word Frau. Notice the pairs arising from the homographs bow, row and sow.
This is a contrast between two diphthongs with similar starting positions and direction of movement. It is a known problem for Scandinavian learners and possibly others, though most learners will simply replace /əʊ/ with a monophthong.
Interesting pairs include:
The density figure is 6.28%. The set makes 70 semantic contrasts giving a loading of 52.2%.
arose arouse beau bough beaux boughs beau bow beaux bows blows blouse boat bout boats bouts bode bowed Boer bower Boers bowers boned bound bow bough bows boughs bow bow bows bows bowing bowing bowed bowed chose chows clone clown clones clowns Co cow coach couch coached couched coaches couches coaching couching coal cowl coaling cowling coals cowls code cowed crone crown crones crowns crowed crowd devote devout dhoti doughty doe dhow does dhows dose douse dosed doused doses douses dosing dousing dote doubt doted doubted dotes doubts doting doubting doze dowse dozing dowsing dozed dowsed dozes dowses drone drown droned drowned drones drowns droning drowning float flout floated flouted floating flouting floats flouts foal foul foaled fouled foaling fouling foals fouls fro Frau goat gout groaned ground gross grouse grossed groused grosses grouses grossing grousing hoe how hoes house hole howl holed howled holes howls holing howling honed hound hose house hosed housed hoses houses hosing housing know now known noun load loud loader louder lode loud lower lour lowered loured lowering louring lowers lours moaned mound most moused no now note knout notes knouts oast oust oasts ousts phoned found poach pouch poached pouched poaches pouches poaching pouching pro prow prole prowl proles prowls prose prows road rowed roe row rose rouse roses rouses rote rout row row rows rows rowing rowing rowed rowed scold scowled sew sow sews sows sloe slough sloes sloughs so sow sow sow sows sows stoat stout strode Stroud though thou tone town tones towns Tony townee tote tout toted touted totes touts toting touting woe wow woes wows
John Higgins, Shaftesbury, December 2009