The houses and signs of the zodiac both run in an anticlockwise direction around the horoscope. The house cusp is the line defining the end of one house and the beginning of the next. For example, the Ascendant is the cusp at the beginning of the first and the Descendant the cusp of the seventh, the Medium Coeli the cusp of the tenth house and the Imum Coeli the cusp of the fourth.
A planet's influence is said to increase if it is close to a house cusp. Planets located just before the cusp of the following house are often counted as being in the house that begins with that cusp. The general rule is that planets situated either in the last sixth of a house or within 5 degrees of the cusp are counted to the next house.
This is not necessarily the case with retrograde planets which are moving away from the cusp.
- How House Cusps Are Calculated (Video David Cochrane, 2014)