The tog is a measure of thermal insulance of a unit area, also known as thermal resistance. It is commonly used in the textile industry and often seen quoted on, for example, duvets and carpet underlay.
The Shirley Institute in Manchester, England developed the tog as an easy-to-follow alternative to the SI unit of m⋅K/W. The name comes from the informal word togs for clothing, which itself was probably derived from the word toga, a Roman garment. The backronym thermal overall grade is also attested.
The basic unit of insulation coefficient is the R, (1 m⋅K/W). 1 tog = 0.1 R. There is also a US clothing unit, the clo, equivalent to 0.155 R or 1.55 tog, described in ASTM D-1518.
A tog is 0.1⋅m⋅K/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 3.0 tog (summer) to 16.5 tog (extra-warm).  The stated values are a minimum; actual values may be up to 3 tog higher. Also, these values assume there is no added duvet cover that can trap air.
A few 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.