Plurals of Nouns ending in F or FE which accept of S only in the Plural.
briefs
chiefs
fiefs
griefs
mischiefs
kerchiefs
neckerchiefs
handkerchiefs
caliphs
caufs
clefs
coifs
delfs
dwarfs
turfs
kerfs
surfs
fifes
strifes
safes
scarfs
waifs
woofs
hoofs
roofs
proofs
reproofs
disproofs
waterproofs
beliefs
reliefs
gulfs