The cult of Śiva and the worship of his emblem, the liṅga, are as old as Hinduism itself. The liṅgas installed in temples are almost always made of stone. They consist of three parts out of which only the third part—called ‘Rudrabhāga’—is seen above the ground. This cylindrical portion alone—termed ‘pūjābhāga’—is to be worshipped. It contains certain lines technically called ‘brahmasūtra,’ without which the liṅga becomes unfit for worship.
See also ŚIVALIṄGA.