Name the time shift at the first point of ambiguity
A reader should recognize the move into the past before interpreting the scene. A heading suffix works for one scene; explicit start and end markers are safer for a sequence.
Make the time shift explicit in a scene heading or action cue, then clearly return to the present when the transition could otherwise be missed.
Direct answer
For a one-scene flashback, name the time shift in the scene heading with TEN YEARS EARLIER, a date, or another specific cue, then make the returning heading equally clear with PRESENT when needed. For a brief image, an action cue can be enough. If more than one scene sits in the past, use a clearly labeled FLASHBACK / END FLASHBACK block so the reader never has to guess which timeline is active.
On the page
EXT. HARBOR ROAD - NIGHT - TEN YEARS EARLIER
A younger Mara watches the ferry pull away.
MARA
Wait.
INT. LAST TRAIN - NIGHT - PRESENT
The closing doors snap Mara back to the present.
Evidence weight
A reader should recognize the move into the past before interpreting the scene. A heading suffix works for one scene; explicit start and end markers are safer for a sequence.
Some writers italicize flashbacks or use transitions such as BACK TO PRESENT. Those devices can help, but they are optional. Typography alone is a weak timeline signal because it may be lost in import or export.
Not one rigid template
Add FLASHBACK, YEARS EARLIER, or a specific date to the scene heading when one scene carries the time shift and the return is obvious from the next heading.
Use FLASHBACK / END FLASHBACK around several scenes or when the present-day frame resumes without a naturally explanatory heading.
A short action cue such as A FLASH OF: can carry a single image without manufacturing a complete scene. Return immediately to the ongoing action.
Reader and handoff friction
Choose for the recipient
Use the least notation that makes the timeline immediate. A reader should not need color, revision marks, or production metadata to understand the transition.
Past-period scenes often affect cast, wardrobe, art, and location breakdowns. Keep full scene headings where those distinctions need to remain visible to production systems.
In Plot, put a one-scene time shift directly in its Scene Heading, as in TEN YEARS EARLIER, and make the returning heading equally explicit when needed. For a longer sequence, use clear Action markers around otherwise normal scenes instead of relying on typography alone.
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Complete, source-backed guides for common screenplay situations
Montage
Signal the montage, organize its visual beats in action, and end it clearly when the reader might not otherwise know where ordinary scene flow resumes.
Scene numbers & production
Leave scene numbers out of an ordinary spec unless requested; add and preserve them when a production needs stable references for breakdowns, schedules, and revisions.
V.O. vs O.S.
Use O.S. for a speaker who belongs to the current scene but is outside the frame; use V.O. when the voice comes from outside the scene's immediate physical action.