Femtometer
A femtometer (fm) is a unit of length in the metric system, equal to one-quadrillionth of a meter (1 fm = 10⁻¹⁵ meters). It is also known as a fermi, named after the physicist Enrico Fermi. Femtometers are used to measure extremely small distances at the subatomic level, such as the size of protons, neutrons, and atomic nuclei. For example, the radius of a proton is about 0.84 femtometers. This unit is commonly used in nuclear physics and particle physics, but it's far too small for everyday measurements.
Nail (Cloth)
A nail is a traditional unit of length used in cloth measurement, equal to 2¼ inches or approximately 5.715 centimeters. It was commonly used by tailors and fabric merchants in England to measure small lengths of fabric.
The term “nail” likely originates from the practice of marking lengths on a measuring stick with notches or nails, making it easy to measure fabric quickly. The nail was a convenient subdivision of the yard, with 1 yard = 16 nails.
Though obsolete today, the nail remains an interesting part of historical textile measurement systems.
Summary:
1 nail = 2¼ inches (5.715 cm)
1 yard = 16 nails
Used historically in tailoring and fabric trade
Originates from notches or nails on measuring sticks
No conversions available for length.