I think Stephen really meant the gray card and the whole 18% is not middle gray thing.
One thing I know for sure about a gray card: If you meter it... and shoot it as meter recommends... it will be properly exposed.
Not to hijack the thread, but I have often wondered if the dull grey (or gray) of my vintage Tamrac Expedition was chosen so that it could act as an in-the-field grey (or gray) card. It works... when in doubt meter the camera bag.
Tom, on Point Pelee, Canada
Ansel Adams had the Zone System... I'm working on the points system. First I points it here, and then I points it there...