Tog is basically a measure of how well duvets (as well as other covers) retain heat. Tog ranges on a scale from 1 to 15, with 1 being the coolest. Note that different fillings will require different weights and thicknesses to achieve the same tog. For instance, high quality down is very warm and will create a high tog duvet without being heavy or bulky, while other fillings like polyester will need to be heavier to retain the same amount of your body heat.