What percentage of rye flour is needed call a loaf rye bread? 20%? 30%? More?
Legally, or what? It would depend on where you were to know whether there are labelling standards.
At the risk of sounding cynical - none at all. If you want to call a burger bun a rye bread who's to stop you?
in the United States bread is called by the non white wheat ingredients that set it apart from a pure white wheat loaf. Where as in Europe, bread is named after the most prominent ingredients. So a rye in America could contain as little as 5% but a rye in Europe contains at least 75% if not more to be called a rye loaf. A 50/50 rye/wheat is called a "mixed" loaf.