In refined brass, polished to perfection is Siva's vehicle, the bull. It has a huge body, a thick neck and sleek sides. The eyes look gentle and the horns hard as diamond. Being Shiva's vehicle it is adorned with ornaments and a rich overcloth. Its linga shaped hump, makes the bull a natural choice to appear on his banner and be his insignia. Its limbs are so folded as to make him sit comfortably. What is most notable is that even in hard metal like brass, there seems flexibility in the loose skin of Nandi's neck and there is tactile quality in the overcloth covering its back.
Devotees at a Shiva's shrine often touch the loins and flanks of Shiva's bull, seeking both its strength and restraint.
This description by Renu Rana.