Fair use rationale for Masters of Chant Chapter II ...
• 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.
Lierne' rib` (lyârn' rĭb`). [ French lierne .] (Architecture) 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. 
In Gothic vaulting, a lierne rib is 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.