Also often indicated as L. or  a corner;

The cusps of the Tenth, Fourth, First and Seventh Houses, or the Medium Coeli, Immum Coeli, Oriens (Ascendant) and Occidens (Descendant) of a Solar or, indeed, of any Celestial Figure: any of the four cardinal points in a Figure, or map, of the heavens; variously relating to the Zenith, or South Vertical; the Nadir, or North Vertical; and the East and West horizons. The Southern, Northern, Eastern, and Western angles are typically described herein.

In Astrology, they are the most important and relevant arcs. Based on the composition of the planets and their elements, planets is incredibly potent for good or bad. The word may refer to the house's shape and location as placed on the square maps used by ancient astrologers. V. The Heavens Map.

Many claim that the Ascendant of either diagram is the most powerful angle, while Ptolemy gives priority to the Midheaven, or Zenith, since at their meridian altitude the heavenly bodies are universally more powerful in their impact than when rising.