Cusp

A cusp is a boundary line in a chart — most commonly the boundary between two houses ("the cusp of the 7th house") or between two zodiac signs ("the cusp of Aries and Taurus"). House cusps depend on the house system chosen (Placidus, Whole Sign, Equal, Koch, etc.) — different systems produce different cusps for the same birth chart, especially for the intermediate houses. The cusp of the 1st house is always the Ascendant; the cusp of the 10th is always the Midheaven (in quadrant systems). "Born on the cusp" colloquially means a Sun very near a sign boundary — modern astrology treats this less mystically than older traditions; a Sun at 29° Aries is still firmly Aries, just with stylistic flavor borrowing from Taurus. House cusps with planets sitting tightly on them are read as especially active points in the chart.