There is no surer way to find out what parties meant, than to see what they have done.

Explanation

(English-origin maxim.) The practical construction placed on an agreement by the parties' own conduct is the best evidence of their mutual intent — actions speak louder than recitals.

Where this applies

• Two neighbors dispute the boundary of their property, but for forty years both have treated the old fence line as the border — their conduct is the best evidence of what they agreed the boundary was.
• A contract's payment terms are ambiguous, but for ten years the buyer has paid quarterly and the seller has accepted without complaint — their course of dealing shows what they meant.
• Spouses disagree about whether their prenuptial agreement covers a vacation home, but they've been splitting costs on it equally for a decade — their actions reveal their mutual intent.