The tog is a measure of thermal resistance of a unit area, also known as thermal insulance, commonly used in the textile industry, and often seen quoted on, for example, duvets.
The Shirley Institute in Manchester, England developed the tog as an easy-to-follow alternative to the SI unit of m2K/W. The name comes from the informal word "togs" for clothing which itself was probably derived from the word toga, a Roman garment.
The basic unit of insulation coefficient is the RSI, (1 m² K / Watt). 1 tog = 0.1 RSI. There is also a clo clothing unit equivalent to 0.155 RSI or 1.55 tog.
A tog is 0.1 m2K/W. In other words, the thermal resistance in togs is equal to ten times the temperature difference (in °C) between the two surfaces of a material, when the flow of heat is equal to one watt per square metre.
British duvets are sold in steps of 1.5 tog from 4.5 tog (summer) to 15 tog (extra-warm). The stated values are minima, actual values may be up to 3 tog higher.
Tog guidelines for duvets are as follows:
Lightweight summer duvet: 3.0 - 4.5 tog
Spring/Autumn weight duvet: 7.0 - 10.5 tog
Winter weight duvet: 13.5 - 15.0 tog
Some manufacturers have marketed combined duvet sets consisting of two duvets; one of approximately 4.5 tog and one of approximately 9.0 tog. These can be used individually as summer (4.5 tog) and spring/autumn (9.0 tog). When joined together using press studs around the edges, or Velcro strips across each of the corners, they become a 13.5 tog winter duvet and as such can be made to suit all seasons.