I trace my confusion to childhood, when my mother said something like this: “If you want to go to the beach today, a perquisite is to make your bed first.”
I knew exactly what she meant, and had no idea that what she actually said wasn’t grammatically correct. Now, I know.
A perquisite is a “perk”–a fringe benefit, a gratuity, a tip, a benefit that comes with the job (health insurance, day care, tuition, a company car). Here is a correct use of perquisite:
Sue felt as though she had entered a parallel universe. Her new boss had just informed her that the perquisites of her position included, but were not limited to, a full health care package, a Prius, a yacht, a solar-powered mansion in Spain, and a $10,000,000 golden parachute.
Prerequisite, on the other hand, is something that is necessary to achieve a certain end or carry out a particular function:
Clear thinking is a prerequisite for clear writing.
Humanities 101 is a prerequisite for Humanities 304.
Now, if you lop off the “pre” in prerequisite, you are left with requisite, which means indispensable. Requisite is sometimes a noun and sometimes an adjective:
After making the requisite number of phone calls, I went to lunch. (adj.)
One of the key requisites of good writing is the willingness to rewrite. (noun)
Alarmed, Ginger noticed that the caterer had failed to deliver the requisite amount of sauerkraut for the imminent polka party. (adj.)
A close relative of requisite is requisition:
Finding only one pencil and two ballpoint pens in the supply room, Ginger promptly submitted a requisition form for two dozen pencils, three dozen ballpoint pens–and one supply room clerk.
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