The /ʊ/ vowel is spelled <u>, <oo>, and <ou> in could. The /ʌ/ vowel is spelled <u> throughout. Notice the homograph putting with its two meanings of "placing" and "hitting a golf ball on the green".
Both vowels are short and central in tongue position, though /ʊ/ is rounded. This is a recognised problem for speakers of Dutch and perhaps some other languages, but with so few minimal pairs, it is not very important. Some native speakers, especially from the north of England, do not make the disinction.
The density figure is 1.022%. The set makes 11 semantic contrasts giving a loading of 55%.
book buck booked bucked booking bucking books bucks bookie Buckie could cud crooks crux foot phut look luck looks luxe put putt puts putts putting putting rook ruck rooked rucked rooking rucking rooks rucks shook shuck stood stud took tuck
John Higgins, Shaftesbury, December 2009