The Collaborative International Dictionary of English v.0.48:

Cadency \Ca"den*cy\, n.
   Descent of related families; distinction between the members
   of a family according to their ages.
   [1913 Webster]

   Marks of cadency (Her.), bearings indicating the position
      of the bearer as older or younger son, or as a descendant
      of an older or younger son. See Difference (Her.).
      [1913 Webster]