underrunner

Beneath, under · (figurative) To go from one side to the other; to progress along a path

runner词根/词干

名词 Noun

The lower of a pair of grinding millstones.
An additional runner added to the bottom of a sledge to protect the main runners in harsh conditions.
A runner (mechanical part intended to guide or aid something else to move) designed to be inserted underneath the item to be moved.