A winter bedcover laid over sheets and blankets, which is the forerunner to our modern comforters and duvets. Granny’s homes always had the best eiderdowns, and always a paper or fabric-lined chest/ottoman at the end of the bed in which they were stored during the summer months.
The eider duck is a large seafaring duck from the northern hemisphere–Europe, Siberia, the North Atlantic and the Farne Islands, where St Cuthbert protected the birds in 676. The breast feathers of the female eider duck make the softest and warmest filling–she uses this down herself to line the nest for her young.
Eider down has largely been replaced by goose down, which is every bit as luxurious and indispensable a filling.
Other than eider down, perhaps silk is the nearest competitor, with the advantage that is non-allergenic.