When buying duvet, make sure that you select the correct tog rating:

4.5 TOG – Summer use
10.5 TOG – Autumn and Spring use or Winter in a very warm house
12 TOG – Winter in warmer houses
13.5 TOG – Winter use in cooler houses
15 TOG – Winter use in a cold environment
All Seasons – A combination of 10.5 tog and 4.5 tog duvets which can be used together or separately to provide ideal warmth for all seasons. The 2 duvets can be attached using the Velcro tabs.