How the lines work
Picture page 23 of the script, scene 14. The script supervisor draws one vertical line per setup, top to bottom, through the part of the scene that setup covered.
14 14A 14B 14C
MAYA | | ~
You kept it? | | ~
| | |
JOE | ~ |
I kept everything. | ~ |
| | | |
Joe folds the napkin, slowly. | | | |
| | | v
MAYA | | ~
That's not an answer. | | ~
v v v
Reading across: the master (14) covers the whole passage with both actors on screen. 14A is Maya's single, so it's straight on her lines and squiggly (off camera) on Joe's. 14B is Joe's single, the reverse. 14C is the insert of Joe's hands, which only covers the napkin moment. An arrow or bar at the bottom shows where the setup ended; a line that stops halfway down the page means that setup only covered part of the scene.
At the top of each line the script supervisor writes the slate and the circled takes ("14A: 2, 4").
The facing page
The page opposite each script page carries the notes: setup, lens, description, takes, timings, and what went wrong in each take. That's the continuity report material, laid out next to the lines it refers to.
Why editors care
When the director asks in the cutting room for "a different read on 'I kept everything,'" the assistant editor looks at page 23, sees that line is covered in 14 and 14B with Joe on camera (and 14A with Joe off camera, which might still be usable audio), and pulls those takes. Without the lined script, someone scrubs through every clip of the scene.
Lining errors editors find later
A line drawn through the whole scene when the setup only picked up from the middle hides a gap in coverage. Forgetting to switch from straight to squiggle misleads the editor about who's on screen. And lining on the wrong revision of the script (white pages when the set is using blue) puts page and line references out of step with the shooting script.
The script supervisor keeps the lined script along with the daily script supervisor report. Slate letters follow the production's convention; see scene numbering rules.