Assertion: coil's shape | /r/HasProperty | hourglass
Facets:
Source sentences:
text: Based on 19th century buggy seat springs, the Bonnell coil has an hourglass shape with a knot at each end, and is known as the original mattress coil.
text: The metal coils in an innerspring mattress are usually an hourglass shape — wider at the top and bottom, but thinner towards the middle.