What is the difference between Type I and Type II CompactFlash?

As with PC Card, the Type I/II designation only has to do with the thickness of the card. Type I cards are 3.3mm thick, while Type II or 5.0mm thick. Both are 43mm in width and 36 mm in height. A Type I card will fit into a Type II slot or adapter, while the converse is naturally not the case.