Five 5 in friends.
Nail sizes are classified in a rather unusual way. Length is defined by the word “penny,” which has its origins in England, where nails were originally named for how much 100 of a given size cost. If, for example, 100 nails cost four pence, they were “four penny” nails.
Today, this terminology corresponds with a nail’s measurement from the head to the tip of the point. The letter “d” is used to denote “penny” (this comes from the word “denarius,” a Latin name for the Roman equivalent of a penny). Two penny, or 2d, nails are 1-inch long, and subsequent measurements are given in ¼-inch increments up to 10d (i.e., 4d nails are 1 ½-inch long, 6d nails are 2 inches long, etc.). After that, the system becomes a little trickier. 12d nails are 3 ¼-inch long, 16d nails are 3 ½-inch long and 20d nails are 4 inches long. From there, each 10d adds another ½-inch, so a 50d nail is 5 ½-inch length.
Source: Home Depot