The /ɑ/ vowel is usually spelled <ar> but we also find <a> in ma and <aa> in baa. The /aʊ/ diphthong is spelled <ou>, or <ow>.
This contrast is between a long vowel and a diphthong. The starting tongue positions are very close and confusions are possible even among native speakers.
Interesting pairs include:
The density value is 1.55%. The pairs make 32 semantic contrasts, giving a loading of 50%.
art out baa bough baas boughs baa bow baaing bowing baa'd bowed baas bows bar bough bars boughs bar bow barred bowed bars bows bra brow bras brows can't count carboy cowboy carboys cowboys card cowed Carl cowl car cow cars cows char chow chars chows darn down darned downed darning downing darns downs dart doubt darted doubted darting doubting darts doubts grass grouse grassed groused grasses grouses grassing grousing ha how lard loud ma mow Mars mows mast moused parch pouch parched pouched parches pouches parching pouching part pout parted pouted parting pouting parts pouts rah row scar scow scars scows spars spouse start stout starter stouter tarn town tarns towns tart tout tarted touted tarting touting tarts touts trance trounce trances trounces unbarred unbowed vase vows
John Higgins, Shaftesbury, November 2009