Sour cherries
Small, deep red cherries with a distinctly tart, aromatic flavour, usually cooked rather than eaten raw.
Fresh sour cherries appear only briefly in high summer at markets. Pitted in jars or frozen, they are in almost every supermarket in German-speaking countries, while in English-speaking countries they are more often found in Eastern European shops or by mail order.
Substitute:
Sweet cherries with a little lemon juice come close but stay sweeter and less aromatic. If you use jarred cherries, reduce the sugar in the recipe.