Pack of four black plastic corner pieces.  These pieces sit in the corner of the plastic base and the metal frame of the cage clicks into the piece, making the cage solid and secure.  Fit the following cages.
  
Ferplast Cavie 60, Ferplast Cavie 15, Ferplast Cavie 80, Ferplast Rabbit 80, Ferplast Rabbit 100, Ferplast Rabbit 100 Double, Ferplast Rabbit 120, Ferplast Rabbit 140.