The axes of the horoscope refer to both the Horizontal Axis between the Ascendant and Descendant and the vertical one connecting the Midheaven (or Medium Coeli) with the Imum Coeli. They are also referred to as the main axes of the horoscope.
Each axis divides the horoscope into two parts: the horizontal axis divides it into the diurnal hemisphere (above) and the nocturnal hemisphere (below), the vertical axis into the Eastern Hemisphere (according to astrological tradition the left-hand side of the chart) and the Western Hemisphere (the right-hand side). Together, these two axes form a grand cross and divide the horoscope into four quadrants. These main axes remain identical irrespective of the house system being used – the only changes made are to the position of the other houses house cusps.
The term axis is often used to refer to only the Ascendant, Descendant, Medium Coeli or Imum Coeli without reference to the opposite end of the axis.
But the important characteristic of any axis is that it always connects two opposing points, as in the case of the Moon's Nodal Axis Moon's Nodes. In addition, any two houses situated in opposition are considered to create an axis: for example, the axis formed by the first and seventh houses is referred to as the I/You Axis, the second and eighth as the axis of substance, the third and ninth as the axis of communication, the fourth and tenth as the parental and vocational axis, five and eleven as the axis of creativity, and finally the sixth and twelfth as the axis of existence and transcendence.