First come, first served / First come, first serve
Different formsThe established expression is first come, first served: those who come first are served first.
First come, first served
Use when
You mean people are served or allocated places in the order they arrive or apply.
Example
“Places are allocated on a first come, first served basis.”
First come, first serve
Use when
Avoid this shortened form when you mean the established expression first come, first served.
Example
“Replace 'first come, first serve' with 'first come, first served'.”
Quick check
first come first serve, first-come-first-serve basis
Usage note