In other words, from threepenny to tenpenny 1/4" is added for each penny, but a twelvepenny nail is 3-1/4" long, a sixteenpenny nail is 3-1/2" long, a twentypenny nail is 4" long.

This is explained as meaning that "tenpenny" nails, for example, cost tenpence a hundred.

Another explanation is that originally 1000 of such nails weighed a pound.

The size of cut nails is usually still so indicated.

Nails are sold by the pound.
The advantages of nails are that they are quickly and easily applied, they are strong and cheap, and the work can be separated, tho with difficulty.