Penny Nails

In rereading 'Instant Boats' I find myself again reminded of a gap in my knowledge. While discussing hammers, Payson refers to "heavy nails, such as 8-penny board nails to 20-penny spikes" and "nails 5- and 6-penny or less". Okay, what the heck are penny nails?
 
In defence of my ignorance, let me tell you I am the son of a carpenter (no J.C. jokes, please) and can claim carpentry skills of my own that are a little beyond basic at least. But I have no idea about penny nails. Is this an American thing? While living in Toronto, Vancouver and Victoria I have always bought nails by length (in inches and fractions of inches) and type (bright common, finishing, galvanized spiral, etc.)
 
Enlightenment, please.
jce