In most of the world, affordable multifamily housing belongs to two basic phyla: public and private. The public may enlist the private sector’s help through a few different mechanisms, and the private may be given a boost by the public. But except for that overlap, each phylum sticks to its own side of the street. In northern Europe, however, that is not strictly the case. There, in one subsection of multifamily, lie cooperative apartments — the property development equivalent of a duck-billed platypus that is two decades away from celebrating its third century of existence.