Annex 2 – Units recognised in international practice as determined by the President under Rule 49(2) (see F‑II, 4.13)

4. Units and names of units permitted in specialised fields only

Quantity	Unit
	Name	Symbol	Value
Vergency of optical systems	dioptre		1 dioptre = 1 m^-1
Mass of precious stones	metric carat		1 metric carat = 2 x 10^-4 kg
Area of farmland and building land	are	a	1 a = 10² m²
Mass per unit length of textile yarns and threads	tex	tex	1 tex = 10^-6 kg.m^-1
Blood pressure and pressure of other body fluids	millimetre of mercury	mm Hg	1 mm Hg = 133.322 Pa
Pressure in the fields of plasma physics and semiconductors	millimetre of mercury	mm Hg	1 mm Hg = 133.322387 Pa
Pressure in the fields of plasma physics and semiconductors	Torr	Torr	1 Torr = 133.322368 Pa
Effective cross-sectional area	Barn	b	1b = 10^-28 m²

The prefixes and their symbols listed in F‑II, Annex 2, 1.3 may be used in conjunction with the above units and symbols, with the exception of the millimetre of mercury and its symbol. The multiple of 10² a is, however, called a "hectare".