Assertion: headphone | /r/HasProperty | developed in late 19th century
Facets:
value: to keep their hands
type: PURPOSE
value: for use by telephone operators
type: PURPOSE
Source sentences:
text: The first headphones were developed in the late 19th century for use by telephone operators, to keep their hands free.
text: The first headphones were developed in the late 19th century for use by telephone operators, to keep their hands free.