[urbanspaceman]

Noun Grammar Rules:
Singular = Lady.
Plural = Ladies.
Possessive Singular = Lady's.
Possessive Plural = Ladies' (or strictly Ladies's: not comfortable nowadays).
But...
Proper noun grammar rules:
Singular = Percy.
Plural = Percys.
Possessive Singular = Percy's.
Possessive Plural = Percys' (or strictly Percys's: not comfortable nowadays).

If the singular and plural both end in an s... answers please.