Which fruit is traditionally used to make classic British marmalade?

Classic British marmalade is traditionally made with Seville oranges.

Seville oranges are bitter oranges associated with Seville, Spain. Their sharp flavor and naturally high pectin content make them especially suitable for a preserve that is cooked with sugar and water until it sets. The peel is usually sliced into the finished spread.

Marmalade differs from ordinary orange jam because traditional definitions emphasize citrus fruit and often include pieces of peel. Although many commercial products use sweet oranges or other citrus fruits, Seville orange marmalade is the classic British style.

The fruit is harvested in winter, and homemade marmalade makers often freeze or preserve the oranges because the short seasonal availability makes fresh Seville oranges difficult to obtain year-round.