User blog comment:Wolf night10/The Final Getsuga Tenshou/@comment-1406057-20100611152523

I always thought that Zanpakuto were kind of hereditary. In the turn back the pendulum arc and sword spirits arc (can't remember the name. Sorry.), Ginrei Kuchiki had a Zanpakuto that looked like Byakuya's. It's possible that families could even share spirits, but it has never really been shown due to the fact that only Isshin and Ichigo are the only shinigami to appear together and fight together in the storyline. Just a theory.