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Prefix · secondary / alongside / incidental

by-

From Old English "bi" (beside, near): signals something secondary or additional, as in byproduct and bystander.

Basic

Written by Bryan López, English teacher · Updated October 2026

What does the prefix by- mean?

By- comes from Old English and means "near" or "beside". It shows something that stays on the side, is secondary or happens as an extra effect of something main. It is one of the few prefixes of Germanic origin that is still used today to create words.

By- is not just the word "by"

The prefix by- and the preposition "by" are cousins, but they rarely behave the same way. Unlike prefixes from Latin or Greek, by- does not follow the usual pattern of scientific words, and its meaning drifts from "side" to "secondary" to "extra" depending on the word. The safest approach is to learn each by- word with its own meaning instead of guessing from the prefix.

How it is used

By- attaches to nouns and verbs to signal something secondary, parallel, or alongside:

by- + product → byproduct
a secondary result produced alongside the main output of a process
by- + pass → bypass
an alternative route that avoids the main road or obstacle
by- + stander → bystander
a person standing to the side of an event without participating

"Bye" (goodbye) shares this root — it was originally short for "God be with ye," where "by" implied closeness. "Bygone" (past/gone by) and "byword" (a saying or emblem) also come from this prefix.

How it is pronounced

by-/baɪ/

Tap the button to hear how the prefix sounds. Each word in the table has its own audio.

Examples

Root word

With by-

In a phrase

productbyproductHeat is an unavoidable byproduct of converting fuel into electricity in most power plants.
passbypassSurgeons performed a triple bypass to restore blood flow around the blocked coronary arteries.
standerbystanderSeveral bystanders filmed the confrontation on their phones but no one intervened to help.
lawbylawThe condo association's bylaws strictly prohibit pets above a certain weight limit.
linebylineThe article carried a byline crediting two investigative journalists for the story.
wordbywordThe restaurant became a byword for impeccable service across the entire region.
gonebygoneShe looked at the old photographs and felt a deep nostalgia for bygone days.

Common mistakes

by- is the same as the preposition "by" (by the river, made by hand)

by- as a prefix means "secondary/alongside," not the spatial preposition

"Bypass" does not mean "pass by" — it means "pass around the main route." The prefix "by-" shifts the meaning to secondary or marginal, even though it shares its origin with the preposition.

A trick to remember it

By- = secondary / alongside. A byproduct comes out on the side of a process; a bystander stands to the side of an event; a bypass goes around the main route. Think "by the side of" and the prefix makes instant sense. This Old English root connects directly to everyday vocabulary.

Practise what you learned

Exercise 1 · Form the word

Exercise 2 · Pick the right one

Someone who witnesses a crime on the street but is not involved in it is called a ___.

Frequently asked questions

What does the prefix by- mean in English?

The prefix by- from Old English "bi" (beside, near): signals something secondary or additional, as in byproduct and bystander.

How do you pronounce by-?

The prefix by- is pronounced /baɪ/. For example, "byproduct".

Can you give an example of a word with by-?

"product" becomes "byproduct". It is a typical example of the by- prefix.

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