Fastener sizing comes up all the time when looking at the code book, manufacturer's installation instructions and general framing questions. Sizing almost always refers to the fastener (nail) in terms of 6d, 8d, where the (d) stands for penny's. Why would a "d" stand for a penny? What's a penny?.....

Here's a bit of history that leads up to the awnser of what's a penny.

The denarius "d" was an old Roman coin, and being the first currency on the island of England was solidly entrenched in the early british culture. The denarius, like all things "money", stiffly resisted change. As the centuries passed, the abbreviation "d" for denarius, eventually became associated with the British coin, the penny.

This system of nail sizing originated in 15th century England, when the "penny" size determined what one paid a blacksmith to forge a hundred nails of that size of nail (you could buy 100 10-penny nails for 10 cents or 100 16-penny nails for 16 cents, etc). This price became obsolete before 1500 AD, but is so entrenched in convention, that its use persists to this day

So how long is a 6 - penny nail? Answer: A 6d nail is 2 inches (5.080 cm) long.

The nail sizes go up one quarter inch (635 cm) from the (6d) for each penny. A 7-penny (7d) nail is 2 and a quarter inches (5.715 cm) long.

I hope this helps demystifies fastener sizing for you, it sure helped me just to know the conversion of 1/4 inch per penny.