Article 421 of Jaafari Personal Status Law

a The classes of heirs by lineage are three: 1 Parents and children, however far down the line of descent. 2 Siblings, however far down the line of descent, and grandparents, however far up the line of ascent. 3 Paternal uncles, however far up the line of ascent and down the line of descent, and maternal uncles, however far up the line of ascent and down the line of descent. b These classes are successive in inheritance, meaning that the second class does not inherit unless no one of the first class exists, and that the third class does not inherit while anyone of the second class exists. Chapter One: Inheritance of the First Class, Being Parents and Children