Short answer. The reading favoring the natural right. Rule 130, Section 19 provides that when an instrument is equally susceptible of two interpretations, one in favor of natural right and the other against it, the former — the interpretation favoring natural right — is to be adopted.
What the law says
When an instrument is equally susceptible of two [ ] interpretations, one [ ] in favor of natural right and the other against it, the former is to be adopted.
Rule 130, Section 19 — Construction in favor of natural right. Read the full provision →
A tie-breaker rule, not a general override
Section 19 applies narrowly — only when an instrument is equally susceptible of two interpretations. It is not a license to override clear language whenever one reading seems fairer; it operates specifically in the genuine tie situation, where the text itself does not clearly favor one meaning over the other. Outside that narrow circumstance, ordinary rules of construction, including the other principles found throughout this part of Rule 130, continue to govern. Courts still exhaust the ordinary tools of construction first — reading the instrument as a whole and considering the parties' evident intent — before concluding that a genuine, irreducible ambiguity actually exists.
The tilt toward natural right
Once that genuine ambiguity exists, and one of the two equally plausible readings favors natural right while the other works against it, the rule directs adoption of the interpretation favoring natural right. This reflects a general interpretive preference for readings that respect fundamental rights over ones that curtail them, when the choice is otherwise a toss-up. Natural rights in this sense are those recognized as inherent rather than merely granted by positive law, such as rights bound up with personal liberty, family relations, or ownership of one's own labor and property, so the tie-breaker leans toward preserving rather than diminishing those interests.
Why the rule is deliberately narrow
Confining this preference to cases of genuine, equal ambiguity keeps it from swallowing ordinary contract interpretation — courts still give effect to clear language even where it works against a party's natural-right interests. The rule steps in only as a last resort when the instrument's own words leave the question truly open. This keeps the rule as a targeted tie-breaker for genuine ambiguity, rather than a general instrument for reshaping contracts whose language is otherwise clear. A party cannot manufacture ambiguity simply by proposing a strained alternative reading; the two interpretations must both be reasonable and roughly equally supported by the instrument's language before Section 19 has anything to say about which one wins.