The /ʊ/ vowel is usually spelled <oo>, <u>, and <oul> in could. The /aʊ/ diphthong is spelled <ou> or <ow>. The computer also found the pairs luring/louring and tour/tower, but I have taken them out since the /ʊə/ diphthong in tour and luring is best treated as a distinct phoneme, and some speakers would use a palatal glide in luring in any case.
This is a contrast between a short vowel and a diphthong, fairly close in their starting positions in the mouth. There is some likelihood of confusion though not associated with any one group of learners.
The density figure is 0.71%. The set makes 6 semantic contrasts giving a loading of 42.9%.
could cowed full foul fuller fouler fullest foulest fullness foulness fully foully fuller fowler fullers fowlers poorhouse powerhouse poorhouses powerhouses put pout puts pouts putsch pouch putsches pouches
Credit: John Higgins