250.50 (ALL) grounding electrodes as described in 250.52(A)1 though (A)6 that are present at each building or structure served (SHALL) be bonded together to form the grounding electrode SYSTEM.
250.52:
A(1) Metal underground Water pipe
(2) Metal frame of the Building
(3) Concrete encaseded Electrode
(4) Ground Ring
(5) Rod and Pipe Elewctrodes
(6) Plate Electrodes
(7) Other metal underground Systems
See examples Soares Grounding book & Nec handbook/