Horseshoe u /ʊ/

oo

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book /bʊk/
hook /hʊk/
foot /fʊt/
wood /wʊd/
good /gʊd/
cook /kʊk/
look /lʊk/
wool /wʊl/
hood /hʊd/
shook /ʃʊk/
took /tʊk/
stood /stʊd/

u

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bull /bʊl/
bush /bʊʃ/
full /fʊl/
pull /pʊl/
push /pʊʃ/
put /pʊt/
bully /ˈbʊli/
bullet /ˈbʊlɪt/
butcher /ˈbʊʧə/
cushion /ˈkʊʃən/
pudding /ˈpʊdɪŋ/
sugar /ˈʃʊgə/

ou

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could /kʊd/
would /wʊd/
should /ʃʊd/