They started as same word, but their meanings have drifted apart over time.
Comedian ISMO on what separates a boot from a trunk
Figuring it out at once
Editor Emily Brewster clarifies the difference.
Is there one standard way?
'Poets laureate'? 'Court-martials'? The curious history of postpositive adjectives in English.
Why don't they call it the Merriam-Webster comma?