Mae Cymal 13 o Fil Cymru’n darparu’r mecanwaith sy’n galluogi’r Cynulliad i sbarduno refferendwm ynghylch a ddylid cael cyfradd treth incwm i Gymru.  Caiff Prif Weinidog Cymru neu un o Weinidogion Cymru gyflwyno cynnig yn y Cynulliad y dylid gwneud argymhelliad i’w Mawrhydi i wneud Gorchymyn i gynnal refferendwm.
Clause 13 of the Wales Bill provides the mechanism through which the Assembly can trigger a referendum on whether there should be a Welsh rate of income tax.  The First Minister or a Welsh Minister may move a resolution in the Assembly that a recommendation should be made to Her Majesty to make an Order to hold a referendum.