The Tribe of Joseph received the birthright of a double portion of their inheritance. In fact, double is exactly what they got... instead of one tribe, Joseph was immediately split in two. Joseph's sons (Ephraim and Manasseh) each received a blessing and inheritance from their grandfather, Jacob (Israel).

It can be confusing to keep track of the tribe due to it splitting, and then seeing one of the sub-tribes (Manasseh) also split. It also doesn't help that the two sub-tribes sometimes go by their father's name... However, just know that, in the end, they are (and will be) the Tribe of Joseph.