This is a very large, flavourful, juicy steak cut from the short loin and sold with a T-shaped bone in it. To be called a Porterhouse Steak, it has to have a portion of the Tenderloin in it no smaller than 1 1/4 inches (3 cm) in diameter. This Tenderloin portion in it is actually a Filet Mignon.
If you remove the Filet Mignon piece at home, you basically have a T-Bone steak. Remove the T-bone, and you have a generously sized strip loin steak. How the Porterhouse cut differs from a T-Bone steak is in the size of the Filet Mignon attached to it: a T-Bone will only have a piece of tenderloin in it from 1/2 to 1 1/4 inches (1 to 3 cm) in size.
The steaks are anywhere from 1 1/4 to 3 inches thick (3 to 7.5 cm.) Get one on the thinner side if you want to pan fry it.
The Porterhouse is the steak that many beef aficionados prefer over filet mignons.
In Canada, the government food agent (CFIA) has recently confused matters by saying that a Porterhouse steak may be referred to as a T-Bone steak, which as you can see from above, it is not.
Literature & Lore
"A mighty porterhouse steak an inch and a half thick, hot and sputtering from the griddle; dusted with fragrant pepper; enriched with little melting bits of butter of the most unimpeachable freshness and genuineness; the precious juices of the meat trickling out and joining the gravy, archipelagoed with mushrooms; a township or two of tender, yellowish fat gracing an out-lying district of this ample county of beefsteak; the long white bone which divides the sirloin from the tenderloin still in its place." -- Mark Twain, while touring Europe in 1878.