The “line” in early modern poetics was a confusing concept due to competing definitions of line length. “Length” could refer to classical, vernacular, or visual measurement. “Length” could figuratively refer to a poet’s “line of life” where a lasting reputation was a measure of a poet’s authority, conflated with the length and measure of his or her lines. Despite the cultural importance of the line, studies of lineation are rare, and few account for the line’s assembly of definitions and vital relationship to poetic authority. This thesis therefore offers an account of lineation and the poetic authority surrounding lineation in editorial and performance traditions. It examines changes to lines in playtexts, songs, and actors’ parts through the seventeenth- and eighteenth-century Shakespearean tradition. It argues that changes in ideas about lineation are both signs and consequences of the continual struggle to adapt Shakespeare’s plays to different performative and textual purposes.