possessives of compound nouns
Placement of the apostrophe in a compound noun.
Add ’s to the last word of the compound: sister-in-law’s car, attorney general’s brief, editor in chief’s note. Form the plural on the main noun: sisters-in-law, attorneys general, editors in chief. The plural possessive then takes the mark at the end: sisters-in-law’s house. If that form is awkward, use an of phrase instead: the brief of the attorneys general.
Examples
Use the commander in chief’s order (the ’s goes on the last word of the compound)
Use both fathers-in-law’s trucks (the plural is on fathers, and the ’s is at the end)
Use the names of the secretaries of state (an of phrase replaces an awkward plural possessive)
Avoid the commander’s in chief order (the ’s can’t go inside the compound)
Avoid the secretaries’ of state names (the apostrophe can’t split the compound)