The Primus (fem. Prima, plural Primi) is the head of one of the Houses of Hermes. All primi have the power to expell a magus from their House, rendering him Orbus. Each primus also has a seat in the Grand Tribunal. Beyond that, the powers and authoriy of the primi is a matter of House traditions. Most primi are effective leaders of their House and are highly respected and powerful figures in the Order.
The manner of their choice is a matter of House traditions. In many Houses, the House gathers to choose a new Primus when needed. In others, it is a matter of succession.