Short answer. Article 660 of the Civil Code lists seven physical signs — like a window in the wall, an uneven or slanting face, the wall sitting entirely within one estate's boundary, or bearing only one building's roof beams. Any one of these signs shifts ownership exclusively to the neighbor the sign favors, instead of joint ownership.
What the law says
It is understood that there is an exterior sign, contrary to the easement of party wall: (1) Whenever in the dividing wall of buildings there is a window or opening; (2) Whenever the dividing wall is, on one side, straight and plumb on all its facement, and on the other, it has similar conditions on the upper part, but the lower part slants or projects outward; (3) Whenever the entire wall is built within the boundaries of one of the estates; (4) Whenever the dividing wall bears the burden of the binding beams, floors and roof frame of one of the buildings, but not those of the others
Civil Code, Article 660 — Exterior Signs Against a Party Wall. Read the full provision →
Why these are called "exterior signs"
Article 660 lists physical features that are, in its own words, an exterior sign, contrary to the easement of party wall. A party wall is ordinarily presumed shared between neighbors, but this article identifies visible, physical evidence pointing the other way — toward exclusive ownership by just one side. These are called exterior signs because they are things you can actually observe about how the wall is built and situated, rather than something that has to be proven through documents or testimony alone.
The signs tied to the wall's construction
Several signs concern how the wall itself was built. There is a sign whenever in the dividing wall of buildings there is a window or opening, since a shared party wall would not typically have an opening cut through it. There is also a sign whenever the dividing wall is, on one side, straight and plumb on all its facement, and on the other, it has similar conditions on the upper part, but the lower part slants or projects outward — an asymmetry in how the wall was constructed on each side.
The signs tied to location and structural load
Two further signs look at where the wall sits and what it carries. There is a sign whenever the entire wall is built within the boundaries of one of the estates, rather than straddling the property line the way a genuine party wall would. And there is a sign whenever the dividing wall bears the burden of the binding beams, floors and roof frame of one of the buildings, but not those of the others — meaning only one house's structure actually rests on the wall.
What finding a sign actually does
Where any one of these signs is present, the article's closing rule applies: the ownership of the walls, fences or hedges shall be deemed to belong exclusively to the owner of the property or tenement which has in its favor the presumption based on any one of these signs. So a single confirmed sign is enough to shift the presumption to exclusive ownership favoring the side the sign points to, displacing the ordinary assumption of joint ownership between neighbors.