They started as same word, but their meanings have drifted apart over time.
Is there one standard way?
What's with his feathered cap?
How to remember which is which
Comedian ISMO on what separates a boot from a trunk
A simple way to keep them apart. (Most of the time.)
'Poets laureate'? 'Court-martials'? The curious history of postpositive adjectives in English.