Depending on which story you believe, the term penny was derived in England based upon the number of pence you paid for a pound of nail. Being that the larger nails cost more you there for have a larger designation.
The “d” is derived from denarious, a roman coin similar to the penny.
The most common nails you see in construction are the 6d, 8d, 12d, and 16d. Nails over 4"- 20d, are commonly referred to as spikes.
The most common problem you find in regards to home inspections and nails are improper sized and wrong application, especially with common nails being used in pressured treated materials.
Those who do phased inspections can easily pick apart a house if they so choose…especially in regards to exterior sheathing, joist and trust fastening.
Your basic 16d cc fastener has a minimum of 200 # of holding power… at least according to an engineer that I work with from time to time… I can also attest that when driven through you hand or foot, which I have done at least a half dozen times with a nail gun… they are difficult to get out…although I have never measured the force it takes to pull one out, it does in deed hurt.