Both the porterhouse and the T-bone come from the loin primal. This is an area just below the backbone toward the rear of the cow, but not quite to the rump.
When your butcher cuts a cross-section from the short loin, the vertebrae form the trademark “T.” On one side of the “T,” the steak is a cross-section of the tenderloin muscle. And on the other side of the bone is what’s commonly called strip steak.