Baleen whales do not have teeth, except as embryos. Fossil adult baleen whales did have teeth, but modern adults have only baleen. The embryo teeth are replaced by baleen, which look like a curtain of long plates hanging down from the top of the whale's mouth. Those plates might be 12 feet long, and a foot or more wide. Looking at them from the outside, they look like straight knives hanging down, but from the inside, they are like a big toothbrush or comb. Baleen is made of keratin, a tissue also found in mammalian hair and nails.