Handkerchiefs is correct.
Get Out Your Handkerchiefs was created on 1978-01-11.
Cylinders.
McDonald's
boats
Martinez's
cobwebs
The plural is handkerchiefs. All you have to do to make "handkerchief" plural is add an "s".
You wouldn't pluralize "processing", but you would pluralize "process" making more than one process "processes".
Handkerchiefs is the plural. The singular form of the word is handkerchief.
Fournesses is the plural of Fourness
Sirs is the plural of sir
Folisis is the plural of Folisi