A dense, dark, sour loaf of all rye, leavened with sourdough and left a day before cutting.
What makes it rye sourdough
All rye, or nearly.
Leavened with sourdough for the acidity.
Baked long, and rested a day before slicing.
Where it comes from
Rye doesn't build gluten the way wheat does; its loaf is held up by starch and gums instead. And rye flour is rich in enzymes that would turn that starch to gum — the acid of a sourdough holds them back. So all-rye bread is sourdough bread by necessity, not style.
Across the north it's the everyday loaf: under smørrebrød in Denmark, in thin rounds with a hole in the middle in Finland, packed with whole grains in Germany.
How it varies
Rugbrød
Danish, with cracked rye and seeds, baked in a tin.
Ruisleipä
Finnish, flatter, often a ring.
Pumpernickel
Westphalian, baked very long at very low heat until nearly black.