Linea reeta
Black's Law Dictionary · Henry Campbell Black, M.A. · 1910
Black's Law Dictionary
The direct line ; the vertical line. In computing degrees of kindred and the succession to estates, this term denotes the direct line of ascendants and descendants. Where a person springs from another immediately, or mediately through a third person, they are said to be in the direct line, (linea reeta,) and are calied "ascendants" and "descendants."
Mackeld. Rom. Law, § 129.