Plural of “passerby”

The plural of passerby is passersby (some dictionaries also accept passerbys).

Why

"Passerby" is a compound built from the verb phrase "to pass by" plus the agent suffix "-er," describing someone who happens to pass a particular spot rather than stopping. Because the noun element "passer" (the one doing the passing) is the true head of the compound and "by" is simply a preposition tacked on afterward, the plural marker attaches to "passer," not to the end of the whole word, giving "passersby" rather than "passerbys."

This head-first pluralization pattern is the same logic that governs "attorneys general" and "courts-martial" elsewhere on this site: English compounds built from a noun followed by a modifying word or phrase pluralize the noun itself, wherever it happens to sit in the compound, rather than simply adding -s to the very end of the whole string. "Passerby" is unusual in that ordinary speakers rarely stop to analyze its internal structure, since it's typically treated as a single, solid word rather than a hyphenated phrase, which is likely why the simpler "passerbys" has crept into increasingly common informal use despite not being the traditionally preferred form.

Usage note

Formal and edited writing, news reporting and legal or witness statements included, should stick with "passersby" ("several passersby reported hearing the crash"). "Passerbys" shows up plenty in casual speech, but anything edited still expects the traditional form.

Legal and police reports rely heavily on "passersby" as a precise term for uninvolved witnesses to an incident, distinct from participants, victims, or bystanders with a direct connection to what happened, a distinction that can matter for how testimony is weighted in an investigation. The word's solid, unhyphenated spelling, unlike the hyphenated "brother-in-law" or "attorney general," likely contributes to why so many writers instinctively pluralize it at the very end; without a visible hyphen marking the compound's internal seams, "passerby" simply looks like one ordinary word rather than a noun-plus-preposition phrase.

Common real mistakes

The most common mistake is writing "passerbys," pluralizing the word as if it were a single simple noun rather than recognizing "passer" as the actual head noun being pluralized. A related, rarer error is "passersbys," doubly pluralizing both parts of the compound, a hypercorrection that shows some awareness of the word's internal structure without quite applying the rule correctly.

The wider pattern

A noun built from a verb plus an agentive suffix, followed by a trailing preposition or adverb, is a genuinely rare shape in English, but it isn't unique: "runner-up" and "court-martial" both put the plural on the noun element too, though their visible hyphens make that internal structure much easier for most writers to spot than it is in the solidly spelled "passerby."

Examples

• Several passersby stopped to help after the accident.
• Witnesses among the passersby gave conflicting accounts to police.
• The street performer drew a small crowd of passersby.
• Two passersby called for an ambulance immediately.
• The charity collected donations from passersby all afternoon.
• Security footage showed passersby reacting to the commotion.
• Curious passersby paused to read the historical plaque.
• The busker's guitar case filled with coins from passersby.
• Several passersby offered directions to the confused tourists.
• The mural attracted passersby who stopped to take photos.
• Reporters interviewed passersby who witnessed the parade route change.
• The vendor thanked passersby who stopped to browse his stall.
• Onlookers among the passersby applauded the street musician.
• A group of passersby gathered to watch the film crew at work.
• Startled passersby watched the parade balloon break free.
• Concerned passersby alerted the shopkeeper about the smoke.
• Amused passersby paused to watch the impromptu dance.
• Bystanders among the passersby snapped photos of the fallen tree.
• Nearby passersby rushed to pull the swimmer from the water.
• The mural drew passersby who paused for a closer look.