User blog comment:Kurosaki tenshou/Relationship between Zangetsu and Engetsu/@comment-5210873-20120619101438

Well I cant really tell for sure... but since Isshin is Ichigo's father, there reiatsu should be similar with each other, which means, that since Isshin was a shinigami, and transfered his reiatsu, or some of it, to Ichigo. And I do believe that Zangetsu out springs from Engetsu, since their names is very similar.

- Just what i think, not sure if its correct.