What is "lierne rib"

Lierne rib \Lierne" rib`\ (ly[^a]rn" r[i^]b`) n. [F. lierne.] (Arch.) In Gothic vaulting, any rib which does not spring from the impost and is not a ridge rib, but passes from one boss or intersection of the principal ribs to another.

n. (context architecture English) In Gothic vaulting, any rib which does not spring from the impost and is not a ridge rib, but passes from one boss or intersection of the principal ribs to another.