The Tog rating describes the warmth or "Thermal Resistance" of a product and is widely used to explain warmth levels of duvets. The lower the tog rating, the lighter the product the higher the rating, the warmer the product. The Tog Rating could be 2-4 which will be equivalent to a 200gsm synthetic fibre filling ideal for room temperature range of 12-20 degrees centigrade approximately. A 400gsm is about 7-9 tog rating while a 600gsm is equivalent to 10-13 tog. These are for lower temperatures.