A double boiler
is a stovetop apparatus used to cook delicate sauces
such as beurre blanc
or to melt chocolate
without burning or seizing. It is a double-decker saucepan
with an upper vessel that fits into a lower pot.
One fills the bottom with boiling water and places the food to be cooked in the top one. The top pot is kept above the boiling water and heated only by steam, but tight fitting of the two pots prevents steam from escaping.