What is the usual pattern of the posterior interventricular artery of the heart?

The usual pattern of the coronary arteries is described in most standard medical school-level anatomy texts and references. One very nice description (including data on variations) is found in Gray's Anatomy, 39th edition, 2005, Susan Standing, (Ed.), Elsevier Churchill Livingstone, pages 1014-1018. The e-edition of this text offers updated, on-line references in addition to the print version. I enthusiastically recommend this reference.

Briefly, in usual right coronary artery dominance (~70%), the posterior interventricular artery arises as the terminal branch of the right coronary artery, and lies in the interventricular groove. It is usually a single vessel, but may be flanked by parallel branches of the right coronary artery. The main variation (~10%) is left coronary dominance, in which the posterior interventricular arises from the left coronary artery. Again, the Gray's Anatomy reference is very nice for the fuller details.