In the line of descent; not collateral; as, a descendant in the direct line.

A character, thus [?], placed at the end of a staff on the line or space of the first note of the next staff, to apprise the performer of its situation.

in a straight unbroken line of descent from parent to child; "lineal ancestors"; "lineal heirs"; "a direct descendant of the king"; "direct heredity"

a simple attack or riposte that finishes in the same line in which it was formed, with no feints out of that line.

Amount of light transmitted through a sample, directly in-line with the incident light, or at a certain angle (usually around 5-10°) from it. The ration of these two values is the Haze which is a measure of diffused light.