Allen wrench
US /ˈæl.ən ˌrentʃ/
UK /ˈæl.ən ˌrentʃ/
1.
a type of wrench used to turn bolts and screws with hexagonal sockets, also known as a hex key
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He used an Allen wrench to assemble the furniture.
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Do you have a small Allen wrench for this screw?