Apostrophe for Indefinite Pronoun Possessive
The Apostrophe
Rulebase Summary
Use an apostrophe and s for any of the indefinite pronouns when they are in the singular possessive. Use s and an apostrophe for indefinite pronouns in the plural possessive. Common indefinite pronouns include: another, anybody, anyone, everybody, everyone, nobody, no one, one, other, somebody, someone, and their else forms.
Morson's Rule 145 · p.85
Examples
✗
A. It was someone's elses responsibility, not mine.
✓
A. It was someone else's responsibility, not mine.
The possessive marker goes on the last element — 'someone else's,' not 'someone's else's.'
✗
A. That is everybodys concern, not just mine.
✓
A. That is everybody's concern, not just mine.
Indefinite pronouns form the singular possessive with apostrophe and s.
Style Guide References (1)
Morson's
Rule 145
Page 85