What this is
mon/ma and ton/ta both agree with the noun they modify — you already know that. son, sa, and ses do exactly the same thing, following exactly the same rule. What makes this concept its own lesson rather than a two-line footnote is what that rule collides with: English “his” and “her” encode the owner’s gender, and French son/sa/ses simply don’t. This is the single most common source of possessive mistakes for an English speaker, and it deserves to be seen in isolation before it gets buried under other material.
The rule
son, sa, and ses agree with the noun that follows — full stop, regardless of who owns it:
son livre his book / her book (livre is masculine)
sa voiture his car / her car (voiture is feminine)
ses amis his friends / her friends (plural)Read that again: son livre and son livre are the same two words whether Paul owns the book or Marie does. The possessive never tells you the owner’s gender — only the noun’s.
Paul cherche son livre. Paul is looking for his book.
Marie cherche son livre. Marie is looking for her book.Same son, same livre, different owner. Nothing in the possessive itself changed, because nothing about the noun changed.
leur and leurs work the same way notre and votre did — no separate feminine form at all, only a plural:
leur maison their house (singular — one house)
leurs maisons their houses (plural — more than one house)Why it works this way
French possessive adjectives were never designed to encode information about the owner — they’re doing the same job as le/la/les, agreeing with the noun in front of them, and that job has nothing to do with who the owner is. English possessives happen to encode owner-gender because “his” and “her” evolved from pronouns that already carried that information; French took a different, and honestly more internally consistent, path: possessives behave exactly like every other adjective in the language, agreeing with what they touch.
Edge cases
son/sa/ses tell you nothing about the owner
This is worth internalizing as a reflex: hearing or reading son, sa, or ses, your first question should be “what’s the gender of the noun,” never “who owns this.” The owner’s identity has to come from context — a name mentioned earlier, a previous sentence, general knowledge of the situation.
When the ambiguity actually matters
If a sentence genuinely needs to specify whose book it is, French doesn’t lean on son/sa/ses harder — it sidesteps the problem with de + the owner’s name: le livre de Marie (Marie’s book) instead of son livre, whenever “his” vs “her” needs to be unambiguous. You’ll meet this properly in the next concept.
leur/leurs mirrors notre/votre exactly
No separate feminine spelling, ever. leur maison and leur chien are identical in form; only leurs marks a change, and that change is about number, never gender.
What English speakers get wrong here
The almost-universal mistake is picking son/sa based on the OWNER’s gender instead of the noun’s — writing sa livre for “her book” because the owner is female, when livre being masculine means it has to be son livre regardless. The fix is procedural: before choosing son or sa, identify the gender of the noun that comes right after it. The owner is irrelevant to that choice.