The lines separating the zodiac and houses in a birth chart are called Cusps. Each house consists of 30 degrees in which the planets travel or transit. The Cusps are simply the beginning of a house and the start of a sign, where one sign ends, and another begins. When the Sun moves from the 30th degree of one sign into the 1st degree of the next, it is the beginning of a new sign and the end of an old one.
A Cusp is not someone born on this boundary line, but rather someone who has one foot in two different signs.
Pop culture has sensationalized this phenomenon, claiming that people born on the Cusp are both signs, which is not entirely true. The moment the Sun (or any planet) shifts from the 30th degree of one sign into the 1st degree of another, it is officially a new sign. The Sun will express the qualities of the new sign more strongly than the sign it's leaving.
