The Regiomontanus house system divides the equator into 12 equal parts, starting from the ascending degree. Then a line is drawn through each point connecting North and South.
Such a dividing line is called a house arc. Its intersection with the plane of the ecliptic measures the cusp of the corresponding house.
If we take a planet and draw a line through it according to the same logic above, then such a line will be called a circle of position.
The intersection of the circle of position with the equator is called the mundane position of the planet. The mundane position's right ascension is called oblique ascension on its horizon. The horizon is the circle of position.