2865: A Non-Standard Unit? Oct 24, 2024

Those partial to the metric system may mock imperial or US customary units for their lack of consistent increments, but the top contender may be the penny, abbreviated: d, historically the abbreviation for pence. It is used for nails, especially those not measurable with wire gauges, but what makes them non-standard is their lack of internal consistency. For instance, while at first they seem to increase by one penny per ¼-inch, this does not hold up.

2d = 1 in., 4d = 1½ in. and 10 = 3in. in length, but that’s when the logical increments stop. Meanwhile 20d = 4in., (not 5 ½”) and 60d is only 6in. and not the staggering 15½ inches one would expect having looked at the beginning of the scale.

This is because, even though nails are still sold in this way (only) as a system of dimensional measurement, the name is a reflection of how much 100 (or 120) nails of such a size cost in 15th century England. That is to say, 2 pennies got a hundred 1-inch nails, but 6 pennies only got a hundred 6-inch nails.